Don't call update_cached_manifests task for hosted apps (bug 945236)

https://bugzilla.mozilla.org/show_bug.cgi?id=945236

Also improve publicise/publicise_version developer pages tests. Similar to the work done for https://bugzilla.mozilla.org/show_bug.cgi?id=960132 except this time more focused on the developer pages. 

The `update_cached_manifests` *task* already has a similar check, but also doing that check before even calling the task should help Celery - it'll have fewer useless tasks to process.