Ignore DB caches.

* Partner information: 3rd party-hosted open edX instance, for an edX solutions client (so will need to get some sort of solution merged in)
* Merge deadline: Nov 4th (soft requirement to minimize code drift, we maintain it on the client fork in the meantime)

These caches cause problems when testing, as the DB schemas across forks may diverge from the mainline. As well, these caches are created upon the first test run if they are absent, so there isn't any obvious need for them to be in the repository.
I would, but that would prevent git from creating the folder upon initializing the repository.

Granted, I haven't tried running the tests to see if the folder is automatically created.