Update .travis.yml for new dependencies

Review Request #7580 — Created Aug. 11, 2015 and submitted — Latest diff uploaded

Information

Djblets
master

Reviewers

Since we now allow Django 1.7 and 1.8 on master, we have to make sure
to install the correct Django version before running setup.py develop
or it will always install the latest version (which is incompatible on
Python 2.6).

Ran successfully on https://travis-ci.org/brennie/djblets