Removing deprecation warnings (fixes #360).

Hello, this patch fixes issue #360.
>reversion/models.py:113: RemovedInDjango19Warning: Model class >reversion.models.Version doesn't declare an explicit app_label and either isn't in an >application in INSTALLED_APPS or else was imported before its application was loaded. >This will no longer be supported in Django 1.9.

Django 1.9 needs an explicit app_label.