UP-4287 Switch groovy compilation to GMavenPlus

https://issues.jasig.org/browse/UP-4287

Switch groovy compilation to GMavenPlus to allow:
- groovy files to depend upon java classes
- groovy unit tests to execute against java classes (should allow for spock-based unit tests as well)
- groovy api docs to be included with java api docs

Currently no groovy classes need this.  This change set came about when trying to create a groovy PersonDirectoryDao class that needed to invoke uPortal java code to do its job.  It was not possible with the maven antrun groovyc compilation process.