Switch models to managed=False (rel 980999)

I need that modification to be able to use DRF with OAuth and previously created tastypie tables in https://github.com/mozilla/olympia/pull/15 (see this comment for details: https://github.com/mozilla/olympia/pull/15#issuecomment-38506050)

**BUT** it will probably have side-effects on Zamboni's install. Should I instead modify Olympia's `requirements` file to point to my own fork?