Fix PendingDeprecationWarning for Django 1.6.2

Django 1.6.2 raises a PendingDeprecationWarning if you use @commit_on_success instead of @atomic. Here's someone else with the same problem: http://stackoverflow.com/questions/22510756/pendingdeprecationwarning-on-django-tastypie

I got the issue on Python 2.7.5, and the poster did on Python 3.3+.