Clean up Travis and Tox test configuration

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.