Fixed Python 3 compatibility, added Django 1.6 to Travis testing

To the future!  This PR fixes https://github.com/stefanfoulis/django-phonenumber-field/issues/33

- Unlike the other attempts at this, I did not add any extra dependencies (e.g., the `six` library)
- I also did not import `six` from Django, since it was not bundled until Django 1.5, and version 1.4 is an LTS release supoprted until at least March, 2015.
- The result is that the tests pass on Python (2.6, 2.7, 3.3, 3.4), and Django (1.4, 1.5, 1.6).