Add support for Django 2.2

[Jkrzy]

Lets add support for the new LTS version of Django, 2.2.

[tbaxter-18f]

The ci config appears to be specifying 1.10, the old LTS

[tbaxter-18f]

Might be a good time to look at #40 too. I don't want us to break this thing for anyone who hasn't updated to Django 2.x

[tbaxter-18f]

From discussions in Slack, we've determined the best path is to align the circle config and setup.py on Django 2.2, update the VERSION to 2.0 and update docs to reflect that anyone wanting still using Django 1.x should use the 1.3.x version.