Add Django 2.0 compatibility

[samifahed]

This adds compatibility to Django 2.0
The change also includes add django2 to tox so it runs the tests against it as well.

[coveralls]

Coverage remained the same at 85.147% when pulling c3eabb9 on Thermondo:master into 2a83c47 on jedie:master.

[codecov]

Codecov Report

Merging #102 into master will not change coverage.
The diff coverage is 66.66%.

@@           Coverage Diff           @@
##           master     #102   +/-   ##
=======================================
  Coverage   78.52%   78.52%           
=======================================
  Files           4        4           
  Lines         433      433           
  Branches      104      104           
=======================================
  Hits          340      340           
  Misses         70       70           
  Partials       23       23

Impacted Files	Coverage Δ
reversion_compare/compare.py	85.99% <66.66%> (ø)	⬆️

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[coveralls]

Coverage remained the same at 85.147% when pulling e0aac3e on Thermondo:master into 2a83c47 on jedie:master.

[jedie]

Great! Thanks for contributing!

[jedie]

Wow, i really forgotten to publish in PyPi :(

I've done this now: https://pypi.org/project/django-reversion-compare/0.8.4/

Thanks for reminder ;)