Address Django 1.11 deprecations

[guillaumep]

In order to prepare the upgrade to more recent versions of Django we need to address Django's deprecation warnings.

To do so, we can run unit tests using the -Wa flag to the python intepreter:

docker-compose run web python3 -Wa /code/souschef/manage.py test

We can also check add the -Wa flag to the Dockerfile to check the warnings when the web server runs.

[lauramarec]

@guillaumep Would that still need to be done?

[guillaumep]

@lauramarec Yes, as it will help us upgrade the software libraries. We do not need that immediately though.