Travis support

This adds a configuration file for Travis CI and fixes a number of issues with testing that came up. I think the repo owner still needs to link their account to Travis and click on a few buttons to fully enable Travis.

I've also removed the version pinning from `requirements.txt` - it made testing multiple versions of Django impossible without downloading it twice (once for the version pinned in `requirements.txt` and then once again for the version specified in the testing matrix)