prevent recursion of version and model saving each other (bug 960044)

http://sentry.dmz.phx1.mozilla.com/addons/marketplace-dev/group/17692/

- app.update(status=amo.STATUS_PENDING)
- The status update triggers Webapp.watch_status which calls update to update the version’s nomination.
- The nomination update triggers Version.update_status which updates app status by calling Webapp.update_status.
- Webapp.update_status calls app.update(status=amo.STATUS_PENDING)