Add factory-boy and mock to core dependencies

They are both used in oscar.test, which means it's "public Oscar API".
Deployments might rely on e.g. the factories for their own test suite,
so all needed dependencies need to be installed as part of Oscar.

factory-boy 2.4 brings some nice, but breaking API changes.

The missing dependencies were discovered when working on
django-oscar-datacash, where some of the old-style factories are used,
but newfactories gets imported into that namespace.