Drop support for old versions of python/django/celery

[winhamwr]

We only need to support:

• django 1.6+
• Celery 3.1+
• python 2.7
• python 3.5

The old versions of django are out of their security support window.

[rhunwicks]

I'd prefer to leave support for Django 1.6 / Python 2.7 for the time being - and I have coded #37 such that both 1.6 and 1.7+ should work without any deprecation warning, and as far as I am aware that is the only area where Django is used.

[winhamwr]

Django 1.6+, it is, then.

[winhamwr]

django-nose bumped from 1.4.3 to 1.4.4 and removed support for Django <1.8. Pegging to 1.4.3 does our test builds good.