Add hook for Groovy compilations to be processed as well

Attempting to use Retrolambda-processed code alongside Spock tests results in incompatible class files being used.

For example:
```
us.eharning.base.SplitObservableTest > initializationError FAILED
    java.lang.UnsupportedClassVersionError: us/eharning/base/SplitObservableTest : Unsupported major.minor version 52.0
```

The general cause:
 * compileGroovy task outputs Java 8 code
 * compileRetrolambdaTest is skipped if no Java test code running, even if present, changes may be missed
 * test executes using older JRE - resulting in failure

This change hooks onto the Groovy plugin and attaches a compileRetrolambdaGroovy* task.
I had to fake the Groovy compilation task name by collecting the Java task name and replacing with Groovy... there's probably a better way but I could not find any relevant Groovy properties on any sourceSet