Change effective on 21 January 2015
The Ruby stack now supports JRuby 188.8.131.52.pre1.
To use JRuby 184.108.40.206.pre1, add this to your Gemfile:
ruby '2.2.0`, :engine => 'jruby', :engine_version => '220.127.116.11.pre1'
The JRuby release notes can be found here.
Note: This version of JRuby is not suitable for production applications. However, it can be used to test that your application is ready for JRuby 18.104.22.168.