Clean up Travis and Tox test configuration

[ryanhiebert]

This should fix #33 as well.

• Use the release version of Django 1.7
• Use the latest patch version of all Django versions
• Simplify Travis DJANGO env variable

Also drastically simplify the tox.ini file.

• Use the new generative envlist feature to drastically simplify the envlist build matrix.
• Organize the envlist by Django version over Python version, since we're more likely to add and drop support by Django version than Python version.
• Use pyXX for the python version of the envlist name, to match the built-in envs from Tox.
• Use Tox factors to avoid creating separate sections for each env in the envlist.
• Let Tox set the basepython automatically from the envlist name, since that is now in the same format as the builtin envs.

[tomchristie]

Wonderfully, thanks!