Fail the tests on deprecation warnings
Fail the tests on deprecation warnings

Since its 1.11 release, Django doesn't report deprecation warnings any
more by default. This means we missed some after the upgrade.

It is especially bad because Django 2.0 will break a lot of things, and
Django 1.11 is meant as a stepping stone towards that, adding lots of
deprecationw warnings to help migrate to 2.0.

With this commit, the deprecation warningsare considered hard failures.

In this way, the CI will help us move towards Django 2.0, or even
upgrade any of our dependencies which handle compatibility breaks with
deprecation periods, the way Django does.

Fixes #817

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