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:

• df61a1a: make all the travis.yml settings the same across modoboa projects.

• 1ffd653: updates for django 1.11 support

  • on_delete will become a required parameter for ForeignKey() set it to the current default CASCADE

  • reverse and reverse_lazy have moved to django.urls

  • MIDDLEWARE_CLASSES is replaced by MIDDLEWARE

• c78c546: 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 #29 into master will not change coverage. The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #29   +/-   ##
=======================================
  Coverage   96.26%   96.26%           
=======================================
  Files          14       14           
  Lines         214      214           
=======================================
  Hits          206      206           
  Misses          8        8

Impacted Files	Coverage Δ
modoboa_imap_migration/migrations/0001_initial.py	100% <ø> (ø)	:arrow_up:
modoboa_imap_migration/tests.py	100% <100%> (ø)	:arrow_up:
modoboa_imap_migration/models.py	92.85% <100%> (ø)	:arrow_up:
modoboa_imap_migration/handlers.py	100% <100%> (ø)	:arrow_up:

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