Add Gemnasium badge

This makes it clear that `mock_redis` has no runtime dependencies.

Change-Id: I814b67f1252285fe16455d9954f46edba3272dde
Reviewed-on: http://gerrit.causes.com/37011
Reviewed-by: Shane da Silva <shane@causes.com>
Tested-by: Shane da Silva <shane@causes.com>