Make flake8 mandatory for tox.
    
    This removes the ignore_errors option, which will cause tox to fail on
    failing flake8.
    
    The migrations (autogenerated) are ignored as well as the ``.tox/``
    directory, which stops the tool from analyzing all of our depednencies
    in all of the virtualenvs.
    
    The test currently passes since #32 has been merged.