Upgrade to JRuby 1.7.3

Jenkins recently upgraded to BouncyCastle 1.47 which created an incompatibility with the JRuby 1.6.5 version used for the Ruby runtime plugin.  JRuby 1.7.3+ is using BC 1.47 as well now but updating the Ruby runtime plugin isn't enough because the Erb.rb now calls force_encoding which was throwing an error when executing ERBs since the output stream didn't have support for that method.