Change effective on 11 June 2015
The Ruby stack now supports JRuby 188.8.131.52.rc1.
To use JRuby 184.108.40.206.rc1, add this to your Gemfile:
ruby '2.2.2', :engine => 'jruby', :engine_version => '220.127.116.11.rc1'
The JRuby release notes can be found on the JRuby website.
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.