Upgrade Django (Django-1.10 no longer supported)

[ghost]

Django 1.10 is now no longer supported, this PR upgrades modoboa to Django 1.11 (the last version of Django to support Python 2). This depends on modoboa/modoboa#1333

I've split this into several patches to make it easier to follow:

• 11776cf: make all the travis.yml settings the same across modoboa projects.

• d40f44f: updates for django 1.11 support

  • reverse and reverse_lazy have moved to django.urls

  • MIDDLEWARE_CLASSES is replaced by MIDDLEWARE

• 584f65a: {} is more efficient than dict()

• d6ddef9: updated test_project, synced with a fresh deployment using Django 1.11.8 and updated settings.py so the tests environment matches a fresh deployment.

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As this depends on modoboa/modoboa#1333 tests on travis will fail until it's merged, I've created a special branch (with altered travis.yml) to prove the tests work upgrade-django-TESTS

[codecov[bot]]

Codecov Report

Merging #36 into master will not change coverage. The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #36   +/-   ##
=======================================
  Coverage   90.28%   90.28%           
=======================================
  Files          13       13           
  Lines         762      762           
=======================================
  Hits          688      688           
  Misses         74       74

Impacted Files	Coverage Δ
modoboa_stats/modo_extension.py	100% <100%> (ø)	:arrow_up:
modoboa_stats/handlers.py	100% <100%> (ø)	:arrow_up:
modoboa_stats/views.py	96.72% <100%> (ø)	:arrow_up:
modoboa_stats/tests/test_views.py	100% <100%> (ø)	:arrow_up:

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