Support Django 1.7

FYI @dhui - altered Travis config to test against the latest patch release of each major Django version.

/review @adepue @dhui 
I think you want `>=`
Also, you might need to quote the installation string so the `>` character doesn't get interpreted by the shell as I/O redirection.
http://pip.readthedocs.org/en/latest/reference/pip_install.html#requirement-specifiers