prevent recursion when calling app.update(status) (bug 960044)

The stack:

- 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 status by calling Webapp.update_status.
- Webapp.update_status calls app.update(status=amo.STATUS_PENDING)
- recurse

I couldn't reproduce this yet in a test, but I'm pretty sure adding that check would stop it from recursing.

http://sentry.mktmon.services.phx1.mozilla.com/mkt/marketplacefirefoxcom/group/3215/events/101466/