Add JRuby 9.2.15.0

Enforce bundler version 1 for ruby 2.3.0 and 2.3.1

Fix ruby#148

Remove bundler enforcement on ruby 2.3

The fix for ruby/rubygems#3570 was released with bundler 2.2.0.rc.1.

TruffleRuby 21.0.0 works on macOS 11.0

Fix error message when trying to install truffleruby on windows

Automatically install libxml2-dev libxslt-dev on TruffleRuby if needed

* This is a special case, because TruffleRuby needs an extra system
  dependency that other Rubies do not, and GitHub runners do not have
  these packages installed by default, while TravisCI has them.

Install specific bundler version when minor and patch versions are gi…

…ven (ruby#139)

Ensure no \r are added when running generate-windows-versions.rb on W…

…indows

Add Ruby 3.0.0

Use an absolute path for `bundle config --local path`

* See ruby#131