Add testing for Django 1.11

[jdufresne]

No description provided.

[codecov]

Codecov Report

Merging #929 into master will increase coverage by 0.18%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #929      +/-   ##
==========================================
+ Coverage   83.07%   83.25%   +0.18%     
==========================================
  Files          31       31              
  Lines        1660     1660              
  Branches      246      246              
==========================================
+ Hits         1379     1382       +3     
+ Misses        200      199       -1     
+ Partials       81       79       -2

Impacted Files	Coverage Δ
debug_toolbar/utils.py	83.73% <0%> (+0.6%)	⬆️
debug_toolbar/middleware.py	90% <0%> (+2.5%)	⬆️

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[matthiask]

Thank you!