Use database cache for testing
Currently, the cache is not flushed between tests.

This can cause false positives where a previous test has been messing with cache keys which a future test uses (eg, wagtail_root_paths)

This commit makes django use the database as the cache which is automatically flushed between tests
Added comment into runtests.py to explain why we're using DatabaseCache
Merge pull request #225 from kaedroho/db-cache

Use database cache for testing