Change effective on 29 October 2014
The Ruby stack now supports JRuby 126.96.36.199 in 2.0.0 (experimental), 1.9.3, 1.8.7 modes.
To use JRuby 188.8.131.52 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.