Delay setting up Django, so e.g. pytest-cov can start coverage first.

Here's my proposed fix for #175. I had to change one test, but since the test didn't explain _why_ it's important for `django.setup()` to be called so early, I don't know if that change in behavior is a problem or not.

Due to #177 I didn't do a full tox run locally, but tests passed on the envs I tried. Hopefully Travis will spot any issues.