Allow tests to pass with Django 2.0

[jdufresne]

Django 2.0 has removed the deprecated setting MIDDLEWARE_CLASSES. Use MIDDLEWARE instead.

As a result, adjust the the expected warnings during test as system check warning 1_10.W001 was removed.

[codecov]

Codecov Report

Merging #993 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #993   +/-   ##
=======================================
  Coverage   83.92%   83.92%           
=======================================
  Files          28       28           
  Lines        1580     1580           
  Branches      236      236           
=======================================
  Hits         1326     1326           
  Misses        188      188           
  Partials       66       66

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[jdufresne]

Given the Django support schedule and the deprecation policy, would it make sense to drop support for Django < 1.11 when adding support for 2.0? I can update this PR to drop all deprecated support if you agree it is a good idea.