Fix metaclass conflict on Django <= 1.6

django.db.models.manager.Manager is an instance of a subclass of `django.utils.deprecation.RenameMethodsBase` during the deprecation cycle of `get_query_set` in favor of `get_queryset`.

See django/django@6983a1a540a6e6c3bd941fa15ddd8cb49f9ec74e for more details.
Merge pull request #2 from charettes/patch-1

Fix metaclass conflict on Django >= 1.6
[Patch courtesy of Simon Charette (@charette) – q.v. http://stackoverflow.com/a/21195171/298171  –ed.]