Decrease slug size when using Git based RubyGems

Change effective on 05 May 2015

When using a Git based RubyGem, the full Git source of the gem is installed on disk. This can sometimes be very large and lead to increased slug sizes. As of now, the .git folder in these gems will be cleaned from disk after a successful deploy. This should lead to decreased slug size when using Git dependencies in a Ruby project. See the Ruby buildpack changelog to view the documentation that has changed.