More Django 1.9 leftovers

[wizpig64]

Follow up to #362

• Removed a pointless MiddlewareMixin alias in simple_history/middleware.py that was part of an old compatibility shim.
• Tests now use the MIDDLEWARE setting rather than MIDDLEWARE_CLASSES. This isn't required until Django 2.0, but now that 1.9 is dropped I see no reason not to switch over now.
• Import reverse() from django.urls rather than deprecated django.core.urlresolvers module.
• Removed usage of django's deprecated @models.permalink decorator. (Deprecated in 1.11, but the fix might as well be done now.)

Thanks :)

[codecov-io]

Codecov Report

Merging #363 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #363      +/-   ##
==========================================
+ Coverage   96.83%   96.83%   +<.01%     
==========================================
  Files          14       14              
  Lines         568      569       +1     
  Branches       70       70              
==========================================
+ Hits          550      551       +1     
  Misses         13       13              
  Partials        5        5

Impacted Files	Coverage Δ
simple_history/models.py	96.35% <100%> (+0.01%)	⬆️
simple_history/admin.py	97.82% <100%> (ø)	⬆️

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[rossmechanic]

lgtm. Thanks for the changes @wizpig64