Add signals documentation

Use django-rest-framework-simplejwt instead of django-rest-framework-jwt

Python 3.7 is now officially supported within Travis CI, so we no longer need to use the development version.

tox-travis will run each Tox configuration for the Python version that it detects, so we don't need to specify the Django versions.

This also removes the "fast finish" setting since we are no longer running a large matrix of builds, so it wont have much effect.

These changes are required since sudo is required for Python 3.7 (for now) and it is only supported on Xenial. Luckily, all of the versions of Python we are testing on works with Xenial.

These versions are supported according to the settings, so let's make sure they are actually supported by running the tests automatically on them.

This appears to be a supported combination in DRF but it was not previously being tested.

Update Travis testing for newer versions

Permissions framework (fix #327)

fix logout Anonymous user