Update URLs for RubyInstaller 1 and DevKit

* See ruby#171
  and oneclick/rubyinstaller#400

Add TruffleRuby 21.1.0

Add reminder to use ruby/setup-ruby@v1 for latest Ruby releases

Automatically pick the latest version of 2.6 for testing

* 2.6.7 is not yet available on Windows

Disable sassc test on JRuby on Windows as it fails on 9.2.17.0

* See jruby/jruby#6648

Document the logic for whether to install Bundler

Add JRuby 9.2.16.0

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.