update for jdk7 compatibility

When I first tried to use the plugin, I ran into an incompatibility with jdk7 (see this post for details - http://blog.proxerd.pl/article/how-to-fix-incompatibleclasschangeerror-for-your-groovy-projects-running-on-jdk7).

This update just converts JenkinsConfigurationException from .groovy to .java.

Thanks for creating this, programmatically maintaining templated job configurations is so much better!