Change effective on 11 December 2013
The Ruby stack now supports JRuby 1.7.9 in 2.0.0 (experimental), 1.9.3, 1.8.7 mode.
To use JRuby 1.7.9 in Ruby 1.9.3 mode, add this to your
ruby '1.9.3', :engine => 'jruby', :engine_version => '1.7.9'
ruby value depending on the mode you wish to use.
The JRuby release notes can be found here.