Fix key sorting bug on database mode 

HI,

I had a problem with the app in database mode, in the admin interface, keys and values are not matching when a new value is saved. 
Some key without modification are displayed as modified and the value from other fields will be displayed...

See this bug report (not from me): https://github.com/comoga/django-constance/issues/31

Feel free to merge it if you think it's the appropriated solution.

Thanks !