Improve build and bump supported versions

[bobwhitelock]

This PR improves and fixes the testing/build setup, and updates the supported Python/Django versions to those that are currently maintained. I made these changes as part of investigating the issue in https://github.com/dstufft/django-passwords/pull/65, as well as confirming to my satisfaction that this library works on more recent Python/Django versions as part of updating my own project. See each commit and the inline comments for full details of the changes being made and why.

You can see an example build for these changes, with https://github.com/dstufft/django-passwords/pull/65 merged as well (otherwise tests would fail), here: https://app.travis-ci.com/github/bobwhitelock/django-passwords/builds/248969588

[darakian]

Not sure you'll get much movement on this. See: https://github.com/dstufft/django-passwords/issues/59

Might be more worthwhile to fork the repo and create a derivative pypi package.

[bobwhitelock]

@darakian Thanks, looks like there has been some recent activity in this repo, so I thought this and #65 might be able to be merged, but no recent releases for some reason. If these can't be merged we'll probably just merge and depend on our fork for now.