Bring up to date with recent versions and conventions

[jdufresne]

• Added testing for Django 1.11 and Python 3.6
• Document supported version in setup.py trove classifiers
• Drop support for outdated, unsupported versions of Django
• Use Travis CI for running tests across all supported versions
• Use Travis CI to run flake8
• Move tests to top level; outside of package
• Correct some Python3 bytes/string mixups
• Add universal wheel support
  • http://pythonwheels.com/
  • https://packaging.python.org/distributing/#wheels
• Update example to work with latest Django

Fixes #25
Fixes #48
Fixes #49
Fixes #50
Fixes #55
Fixes #57

[jdufresne]

To take advantage of the new .travis.yml file, you'll need to sign in at https://travis-ci.org/ and enable the repository.

[alasdairnicol]

@jdufresne thanks for your work on this patch, especially the fix for #49 👍

[pawelad]

@johnsensible would it be possible to get this reviewed/merged?

[mpasternak]

@jdufresne hmm, I just realised I repeated your work in my branch. Oh well. @johnsensible , are you willing to merge stuff? People are using this package. Or, you can pass the maintainer ship to some organisation...