Change effective on 24 January 2014
The Ruby stack now supports JRuby 1.7.10 in 2.0.0 (experimental), 1.9.3, 1.8.7 mode.
To use JRuby 1.7.10 in Ruby 1.9.3 mode, add this to your
ruby '1.9.3', :engine => 'jruby', :engine_version => '1.7.10'
ruby value depending on the mode you wish to use.
The JRuby release notes can be found here.