Fixes test runner on travis-ci.org.

Fix .travis.yml and tox.ini discrepancies.
Add django 1.6 to tox (and kept django-dev).
Remove unsupported python 2.6 from tox.
Remove incompatable combos, like python 3 + django 1.5.

See https://travis-ci.org/georgedorn/django-tastypie for the all-green CI board.