Cleanup current_version and latest_version

This updates the logic so that only apps with a status in
`amo.WEBAPPS_APPROVED_STATUSES` have a `current_version` since a
`current_version` signifies the publicly available version of an app.
Ah, I think i know what was going on... index_webapps was getting called twice b/c of the signal but only executed once b/c of the celery request finished stuff. I dropped a `pdb` there and it only got thrown once. Anyway, this signal makes these tests happier.