Django 1.11.5 upgrade

We are updating to the latest version of the new LTS release of Django, 1.11, which is currently on patch 1.11.5. The changes are documented and explained here: https://bugs.ligo.org/redmine/issues/5478, including necessary package updates for the virtualenv.

This update is live on gracedb-dev1 and has passed all of the unit tests from the current version of the client code.