Most real-world instances will use postgresql instead of sqlite so it's
better to use postgres for the tests. This also allows us to easily add
tests for concurrency issues which is not supported by sqlite

travis: Run py.test --cov=oscar instead of coverage run

I'm not sure why @codeinthehole, back in the days,  opted to
manually configure settings instead of using a regular settings file for
the test suite. Most likely the reasons for doing so have disappeared.

So I turned our tests/config.py into a normal settings file, and use
that accordingly. That means we do things the standard Django way, and
it's easier to drop into a session with the test settings.