Drop support for Django 1.4

I grepped through the code base and removed the obvious candidates.

Three issues remain:
- I'm unsure about the comment in https://github.com/tangentlabs/django-oscar/blob/master/oscar/apps/offer/models.py#L176. Feedback by @codeinthehole needed.
- I wouldn't bother with ``verify_exist`` handling in https://github.com/tangentlabs/django-oscar/blob/master/oscar/models/fields.py#L34 (and the matching form field) because it'll get revisited in https://github.com/tangentlabs/django-oscar/issues/824
- We need to still decide if and how to handle dropping support for Django's old user profile features