[bug 994070] Update purge_hashes for django 1.6

In django 1.6, instead of having a constant unusable password, it now has a unusable password prefix (`!`) followed by random chars.

To test, run `./manage.py purge_hashes`. It takes a bit to run (30 minutes or so maybe?) the first time, because it has to set 600k+ user's passwords to the new unusable.

r?