Change effective on 29 October 2014
The Ruby stack now supports JRuby 188.8.131.52 in 2.0.0 (experimental), 1.9.3, 1.8.7 modes.
To use JRuby 184.108.40.206 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.