Fixes #62 with upgrade to JRuby 1.7.3

Jenkins recently upgraded to BouncyCastle 1.47 which caused an incompatibility with JRuby 1.6.5.  JRuby 1.7.3 has been updated to the same BC version.

The version of stapler-jruby is currently set to 1.208-SNAPSHOT because this fix is dependent on the https://github.com/stapler/stapler/pull/14 due the change 
in JRuby 1.7.3 Erb calling force_encoding.  

Ruby plugins will need to update to jenkins-plugin-runtime 0.2.3 to be compatible with JRuby 1.7.  I'm not sure if there's a cleaner way to handle this but I had to hand edit my config.xmls that were using vagrant-plugin because they were referencing Jenkins::Plugin::Proxies::Builder instead of Jenkins::Tasks::BuilderProxy.

Ruby plugins using jruby-openssl will need to upgrade to 0.8.4+.