Add in workaround for Django 1.4 and six

Borrowed strategy from Django-polymorphism (thanks @vdboor!). Running
this with Django 1.4 the meta class is not switched out properly without
this workaround.

Refs:
https://github.com/chrisglass/django_polymorphic/blob/4be6d42fad60c3e97e4d37f3fc9125d623468908/polymorphic/base.py