Improve .travis.yml.

* Use the latest incremental version of Django
  (1.4.x, 1.5.x, 1.6.x).
* Get rid of some -q flags so we can see what's happened
  when it fails.
Fix test behaviour on Django < 1.6.

* While it may have appeared that django-admin-sso was
  extremely thoroughly tested, with a battery of 484
  tests, all but 6 of those were Django's or OpenID's -
  i.e. those of django-admin-sso's dependencies.
* DiscoverRunner can fix this for us, but it's not
  built into Django < 1.6, so we need to manually
  import it.  The input hacking has been isolated
  in a test runner file (which also makes it easier
  to expand the runner later if we so desire).
* Move tests.py into the newly created test folder
  alongside the test runner and rename it to
  something more descriptive.
Add coverage.

* It needs to be installed and configured. setup.py does
  the former and .coveragerc the latter.
Make Travis run the coverage tests as well.
Yaml tweaks to appease Travis (hopefully)
Move test requirements into their own file.