Made tests work for django<1.5

Also explicitly specify the required django version.
It's obvious that support for django 1.3 is dropped (look at https://github.com/django-mptt/django-mptt/commit/9068e148af4fb091275ea945542fc1fed896231a)

However, the django.utils.six is only available with django>=1.4.2 (see third paragraph from https://docs.djangoproject.com/en/dev/topics/python3/#philosophy)