Fixes #62 with upgrade to JRuby 1.7.3

Fixes #62 
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+.