Fix Django 1.5 User model importing

I get a circular import problem in my Django 1.5 app caused by waffle getting the user model incorrectly.

From the [docs](https://docs.djangoproject.com/en/1.5/topics/auth/customizing/#django.contrib.auth.get_user_model):

> When you define a foreign key or many-to-many relations to the User model, you should specify the custom model using the AUTH_USER_MODEL setting.

`waffle.compat.User` is used to define a `ManyToManyField`.