Pin Django version for nosedjango compatibility

Until nosedjango works with Django 1.6, this is the simplest fix.
Django 1.6 requires a CACHE key in settings

This is just future proofing – if tests are to ever run with Django 1.6,
this needs to be present.
Merge pull request #62 from moeffju/feature/fix-travis-temp

Fix Travis build / fix tests (temporary fix)