Change effective on 29 October 2014
The Ruby stack now supports JRuby 220.127.116.11 in 2.0.0 (experimental), 1.9.3, 1.8.7 modes.
To use JRuby 18.104.22.168 in Ruby 1.9.3 mode, add this to your
ruby '1.9.3', :engine => 'jruby', :engine_version => '1.7.16'
ruby value depending on the mode you wish to use.
The JRuby release notes can be found here.