call save instead of update in pingback api (bug 962717)

eviljeff reported 2 issues:

- Before the push, some apps were having trouble getting to the pending state.
- After the push, 20+ apps don't have a nomination date set.

We fixed the first issue:

- Before the push, we were getting a [recursion error](http://sentry.dmz.phx1.mozilla.com/addons/marketplace-dev/group/17692/) while updating the app status, causing some apps to not achieve pending state.
- Fixed by https://github.com/mozilla/zamboni/pull/1645 where we checked whether the app was already pending first. In the pingback, we set (but not saved) the app's status to pending in https://github.com/mozilla/zamboni/commit/51845d2f2a796911b650521e181ea542c81d56f9 to make sure this check passed.

However, the second issue:

- Somehow the version's signal to update the nomination date isn't being called or isn't being completed.

**Solution:**

Use save instead of update to

- still prevent the recursion since app.status is set before we save and we keep the is_pending check
- still have the version nomination post_save get triggered by calling save.