Remove leftover naive datetimes in tests

Oddly enough, the warnings were only thrown under Python 2.6. Should
investigate!
Test suite: Treat RuntimeWarnings as errors

This ensures we catch e.g. the "naive datetime" RuntimeWarning that
Django throws.
Ideally, we'd fail on all Warnings, but a quick look at implementing
this showed that there's quite a few warnings we have no control over.
Might not be worth the effort of constantly whitelisting them.
Transifex update
Update authors file
Include Bootstrap upgrade in release notes
Fully drop support for Django 1.4
Bump travis Django versions to latest on each branch
Bump version to 0.7.0 final

Woop.