after_commit fixes

I was having issues with the local scaler leaving the delayed_job script running after all jobs were processed. Changing the conditional in the down method didn't have any affect. It appears that the problem was related to having multiple after_commit callbacks using the same method (self.class.scaler.down). As soon as I combined the two into a single after_commit callback, it worked.

I also bumped the rvm version to 1.9.3, which is required by Rails/AR 4.