PLAT-1998 Update test runner and dependency management

[jmbowman]

The tests weren't running under Django 1.11 and there wasn't a good mechanism for testing support of multiple Django versions, so I made a few updates based on what we're now doing in other edX packages:

• Manage dependencies with pip-tools
• Switched from nosedjango to pytest as the test runner
• Added support for using tox to test against multiple Django versions in Travis

I also laid the groundwork for linting import order with isort and checking for Python 3 compatibility issues, but am leaving those disabled so the changes needed to make them pass can be tackled in future PRs. .pylintrc should also be updated via edx-lint later in a separate PR which also fixes any new issues reported by doing that.

[doctoryes]

It'd be easier to shift this repo over to the edx org if we uploaded the wheel to PyPI and started using that pkg in edx-platform. Is that out-of-scope of this PR?

[jmbowman]

@doctoryes Not sure. That path would require us to update the URL in setup.py after the initial release, but that may not be a problem. At any rate, I probably won't have time to tackle it before I head out on PTO tomorrow.

[doctoryes]

@jmbowman Probably won't be a problem - I forgot that GitHub redirects if you transfer a repo between users/orgs.