Added supported python3 versions to travis.yml, updated Django versions, removed system_site_packages

- The classifiers for Python 3.2, 3.3, and 3.4 are already in `setup.py`, so I thought it would be nice to include them in testing.
- I updated the Django version numbers used in testing to the latest stable releases, and added Django 1.6 to the mix.
- I removed `system_site_packages: true` because of this issue: https://github.com/travis-ci/travis-ci/issues/2219