Upgrade django version used to a supported version

[gregcorbett]

Django is currently pinned at 1.6.3.

This was because that was the highest version that worked under Python 2.6. Django 1.6.3 is no longer supported or documented, so we should upgrade to at least a documented version but preferable a supported version.

We no longer need to worry about Python 2.6 support as the docker image is built upon CentOS 7 and as a result uses Python 2.7