I attempted to install 9.2.0.0 on a bunch of systems, including Debian 8 and 9.

On Debian 9 which provides openjdk 8, installation succeeded.

On Debian 8 which provides openjdk 7, installation failed with the following message:

 [2018/07/12 00:55:04.613] Installing jruby-9.2.0.0...
 [2018/07/12 00:55:04.613] BUILD FAILED (Debian 8.1 using ruby-build 20180618-5-gad8f3a4)
 [2018/07/12 00:55:04.615] Inspect or clean up the working tree at /tmp/ruby-build.20180712045503.32377
 [2018/07/12 00:55:04.615] Results logged to /tmp/ruby-build.20180712045503.32377.log
 [2018/07/12 00:55:04.616] Last 10 log lines:
 [2018/07/12 00:55:04.616] jruby-9.2.0.0.tar.gz
 [2018/07/12 00:55:04.616] /tmp/ruby-build.20180712045503.32377/jruby-9.2.0.0 /tmp/ruby-build.20180712045503.32377 /data/mci/bf500ceccf733ed3042569a054af6151/mongo-ruby-toolchain
 [2018/07/12 00:55:04.616] Exception in thread "main" java.lang.UnsupportedClassVersionError: org/jruby/Main : Unsupported major.minor version 52.0
 [2018/07/12 00:55:04.616] 	at java.lang.ClassLoader.findBootstrapClass(Native Method)
 [2018/07/12 00:55:04.616] 	at java.lang.ClassLoader.findBootstrapClassOrNull(ClassLoader.java:1078)
 [2018/07/12 00:55:04.616] 	at java.lang.ClassLoader.loadClass(ClassLoader.java:419)
 [2018/07/12 00:55:04.616] 	at java.lang.ClassLoader.loadClass(ClassLoader.java:417)
 [2018/07/12 00:55:04.616] 	at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:323)
 [2018/07/12 00:55:04.616] 	at java.lang.ClassLoader.loadClass(ClassLoader.java:363)
 [2018/07/12 00:55:04.616] 	at sun.launcher.LauncherHelper.checkAndLoadMain(LauncherHelper.java:482)
 [2018/07/12 00:55:04.641] Command failed: command [pid=28119] encountered problem: exit status 1

Installing java 8 on Debian 8 allowed 9.2.0.0 installation to succeed.

I then attempted to figure out what java is required by jruby:

• http://jruby.org/2018/05/24/jruby-9-2-0-0 does not state any java/jre requirements.
• https://github.com/jruby/jruby/blob/master/BUILDING.md currently states that java 7 or higher is required.

So, I checked out master and attempted to build it on Debian 8, and received the following error:

debian-8% ./mvnw 
[INFO] Scanning for projects...
[WARNING] 
[WARNING] Some problems were encountered while building the effective model for org.jruby:jruby-core:jar:9.2.1.0-SNAPSHOT
[WARNING] 'build.pluginManagement.plugins.plugin.(groupId:artifactId)' must be unique but found duplicate declaration of plugin org.apache.felix:maven-bundle-plugin @ org.jruby:jruby-parent:9.2.1.0-SNAPSHOT, /home/vagrant/jruby/pom.xml, line 338, column 17
[WARNING] 
[WARNING] Some problems were encountered while building the effective model for org.jruby:jruby-stdlib:jar:9.2.1.0-SNAPSHOT
[WARNING] 'build.pluginManagement.plugins.plugin.(groupId:artifactId)' must be unique but found duplicate declaration of plugin org.apache.felix:maven-bundle-plugin @ org.jruby:jruby-parent:9.2.1.0-SNAPSHOT, /home/vagrant/jruby/pom.xml, line 338, column 17
[WARNING] 
[WARNING] Some problems were encountered while building the effective model for org.jruby:jruby-parent:pom:9.2.1.0-SNAPSHOT
[WARNING] 'build.pluginManagement.plugins.plugin.(groupId:artifactId)' must be unique but found duplicate declaration of plugin org.apache.felix:maven-bundle-plugin
[WARNING] 
[WARNING] It is highly recommended to fix these problems because they threaten the stability of your build.
[WARNING] 
[WARNING] For this reason, future Maven versions might no longer support building such malformed projects.
[WARNING] 
[INFO] ------------------------------------------------------------------------
[INFO] Reactor Build Order:
[INFO] 
[INFO] JRuby
[INFO] JRuby Core
[INFO] JRuby Lib Setup
[INFO]                                                                         
[INFO] ------------------------------------------------------------------------
[INFO] Building JRuby 9.2.1.0-SNAPSHOT
[INFO] ------------------------------------------------------------------------
[INFO] 
[INFO] --- maven-enforcer-plugin:1.0:enforce (default) @ jruby-parent ---
[INFO] 
[INFO] --- maven-enforcer-plugin:1.0:enforce (enforce-maven) @ jruby-parent ---
[INFO] 
[INFO] --- maven-install-plugin:2.4:install (default-install) @ jruby-parent ---
[INFO] Installing /home/vagrant/jruby/.polyglot.pom.rb to /home/vagrant/.m2/repository/org/jruby/jruby-parent/9.2.1.0-SNAPSHOT/jruby-parent-9.2.1.0-SNAPSHOT.pom
[INFO]                                                                         
[INFO] ------------------------------------------------------------------------
[INFO] Building JRuby Core 9.2.1.0-SNAPSHOT
[INFO] ------------------------------------------------------------------------
[INFO] 
[INFO] --- maven-enforcer-plugin:1.0:enforce (default) @ jruby-core ---
[INFO] 
[INFO] --- maven-enforcer-plugin:1.0:enforce (enforce-maven) @ jruby-core ---
[INFO] 
[INFO] --- exec-maven-plugin:1.2.1:exec (default) @ jruby-core ---
[INFO] 
[INFO] --- buildnumber-maven-plugin:1.2:create (jruby-revision) @ jruby-core ---
[INFO] Checking for local modifications: skipped.
[INFO] Updating project files from SCM: skipped.
[INFO] ShortRevision tag detected. The value is '7'.
[INFO] Executing: /bin/sh -c cd /home/vagrant/jruby/core && git rev-parse --verify --short=7 HEAD
[INFO] Working directory: /home/vagrant/jruby/core
[INFO] Storing buildNumber: 7026bae at timestamp: 1531430917098
[INFO] ShortRevision tag detected. The value is '7'.
[INFO] Executing: /bin/sh -c cd /home/vagrant/jruby/core && git rev-parse --verify --short=7 HEAD
[INFO] Working directory: /home/vagrant/jruby/core
[INFO] Storing buildScmBranch: UNKNOWN
[INFO] 
[INFO] --- maven-resources-plugin:2.6:resources (default-resources) @ jruby-core ---
[INFO] Using 'utf-8' encoding to copy filtered resources.
[INFO] Copying 41 resources
[INFO] Copying 2 resources
[INFO] Copying 1 resource to /home/vagrant/jruby/core/src/main/java
[INFO] Copying 4 resources to /home/vagrant/jruby/core/target/classes/META-INF/
[INFO] 
[INFO] --- maven-compiler-plugin:3.3:compile (anno) @ jruby-core ---
[INFO] Compiling 7 source files to /home/vagrant/jruby/core/target/classes
[INFO] -------------------------------------------------------------
[ERROR] COMPILATION ERROR : 
[INFO] -------------------------------------------------------------
[ERROR] javac: invalid target release: 1.8
Usage: javac  
use -help for a list of possible options

[INFO] 1 error
[INFO] -------------------------------------------------------------
[INFO] ------------------------------------------------------------------------
[INFO] Reactor Summary:
[INFO] 
[INFO] JRuby .............................................. SUCCESS [  0.303 s]
[INFO] JRuby Core ......................................... FAILURE [  1.638 s]
[INFO] JRuby Lib Setup .................................... SKIPPED
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 7.066 s
[INFO] Finished at: 2018-07-12T21:28:37+00:00
[INFO] Final Memory: 30M/72M
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.3:compile (anno) on project jruby-core: Compilation failure
[ERROR] javac: invalid target release: 1.8
[ERROR] Usage: javac  
[ERROR] use -help for a list of possible options
[ERROR] -> [Help 1]
[ERROR] 
[ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.
[ERROR] Re-run Maven using the -X switch to enable full debug logging.
[ERROR] 
[ERROR] For more information about the errors and possible solutions, please read the following articles:
[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/MojoFailureException
[ERROR] 
[ERROR] After correcting the problems, you can resume the build with the command
[ERROR]   mvn  -rf :jruby-core

... leading me to believe that java 8 is the minimum required version for 9.2.0.0.