Merge pull request #59 from ysb33r/master

Fixes #57, #58
Fix gem reinstallations (bad path) and support gems with java extensions (e.g. thread_safe)

This was tested manually with another project, I'm not unfortunately not
certain how to write good Groovy tests for this

Fixes #61
Make the JRubyExec `script` argument optional provided `jrubyArgs` is set

Fixes #62
Merge pull request #63 from rtyler/62-optional-script-jrubyexec

Make the JRubyExec `script` argument optional provided `jrubyArgs` is present