Add support for Django 2.0

[jdufresne]

Add Django 2.0 to the Travis and tox test matrix.

Add Django 2.0 to the list of trove classifiers.

Update TaggableRel for internal API differences of Django 2.0. Guard with a VERSION check where necessary.

[jdufresne]

The test failure is due to the master branch, not Django 2.0. A fix for the travis configuration is in #504.

[aaugustin]

Thanks, this branch works well, I'm running it in production now.

[hggh]

@alex could you please merge this Django 2 PR into your master and release a new version of taggit?

[codecov-io]

Codecov Report

Merging #509 into master will decrease coverage by 1.64%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #509      +/-   ##
==========================================
- Coverage   89.59%   87.95%   -1.65%     
==========================================
  Files           8        5       -3     
  Lines         673      548     -125     
  Branches      102       96       -6     
==========================================
- Hits          603      482     -121     
+ Misses         55       52       -3     
+ Partials       15       14       -1

Impacted Files	Coverage Δ
taggit/managers.py	89.27% <100%> (+0.41%)	⬆️
taggit/migrations/__init__.py
taggit/models.py
taggit/forms.py

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 294fe61...10a2b81. Read the comment docs.

[nemesifier]

@jdufresne I've merged #504 and restarted the build

[nemesifier]

Latest versions of Pytest dropped support for python 3.3, so that build was failing, I pinned down the version of pytest, let's wait for https://travis-ci.org/alex/django-taggit/builds/319130342 to finish.

[nemesifier]

Ok the python 3.3 build passed after downgrading pytest, I feel confident in merging this now.
BTW I wouldn't mind in dropping support for python 3.3, but I think that decision is not up to me. I'm only an occasional maintainer and I want to operate with the broadest consensus as possible.

[hggh]

@nemesisdesign thanks for merging. python 3.3 is eol: https://www.python.org/dev/peps/pep-0398/#x-end-of-life so it's save to drop it.

There is a PR #512

[nemesifier]

Ok let's do it