Upgrade to Django 1.7c3, new database migrations

Not sure why but this upgrade caused database migrations - I presume the
detection code must have changed slightly. Test this on staging before
deploying to live.

https://www.pivotaltracker.com/story/show/77381344 [Delivers #77381344]
Merge pull request #89 from sealevelresearch/upgrade-django-to-rc3

Upgrade to Django 1.7c3, new database migrations