Fix Django 1.11 deprecations

[MauricioFauth]

Those are the deprecations reported by the tests:

• Old-style middleware using settings.MIDDLEWARE_CLASSES is deprecated. Update your middleware and use settings.MIDDLEWARE instead. (Fixed by bfef7889bcc8b0a961db0a46a4b1a2c2b4591665)
• Importing from django.core.urlresolvers is deprecated in favor of django.urls.
• on_delete will be a required arg for ForeignKey in Django 2.0. Set it to models.CASCADE on models and in existing migrations if you want to maintain the current default behavior.
• permalink() is deprecated in favor of calling django.urls.reverse() in the decorated method.
• Passing a 3-tuple to django.conf.urls.include() is deprecated. Pass a 2-tuple containing the list of patterns and app_name, and provide the namespace argument to include() instead.

[codecov[bot]]

Codecov Report

Merging #156 (d39936e) into master (bfef788) will not change coverage. The diff coverage is 93.33%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #156 +/- ## ======================================= Coverage 63.61% 63.61% ======================================= Files 65 65 Lines 1432 1432 ======================================= Hits 911 911 Misses 521 521 ```