Stub generation doesn't ouput files for all supported Groovy versions

[keeganwitt]

Groovy versions before 1.8.2 don't output stub files. No exceptions are being thrown, the Groovy classes just aren't outputting any files.

[keeganwitt]

I think the only thing we can do (unless we start include more custom fixes -- which we might do as part of issue #2), is to up the minimum supported version to 1.8.2 and let users override the version of Groovy the plugin uses. This is now in master.