Fix loading of Ruby plugins.

In Jenkins 1.519 a change was made to move plugin classes into WEB-INF/lib/classes.jar rather than unpacked into WEB-INF/classes/.

This change causes the ruby runtime to unpack the classes.jar file to WEB-INF/lib/classes/ when it loads a ruby plugin. It also changes the load paths to look for the extra /lib in the file path.

I tested this with the RVM plugin, and it successfully installed the plugin, unpacked the classes.jar file, allowed me to set up a project with RVM, and successfully loaded that project after restarting Jenkins with no data loss.

Fixes [JENKINS-18528](https://issues.jenkins-ci.org/browse/JENKINS-18528).