Fix storage.prefix doesn't exist in Django 1.7

So many unit tests are broken in Django 1.7 since commit
187a7a72bf72370c739f3675bef84532e524eaf1. This patch fixes it.
Unfortunately, we still have to support Python 2.6 and OrderedDict wasn't added until Python 2.7.

However, it doesn't seem like you are using found_files in the code, so maybe we can just get rid of it?