Merge pull request #30 from ysb33r/master

Proposed changes to the handling of default repositories
Refactor and tidy up customizing gemInstallDir while keeping it in project.buildDir by default

See #28
Merge pull request #31 from ysb33r/master

Rolled back to gradle-2.0-bin.zip. Don't know why it changed to -all.zip
Added JRubyPrepareGemsSpec. Fixed transitive dependencies for GEMs. Lots of refactoring.