SW Audit: Verify that new index.html content will be fetched and served

@paulirish and I ran into this on caltrainschedule.io

While we thought our SW was operating correctly, it was always satisfied by the assets in the cache. As a result, it never went looking for a new asset from the network and the user only saw stale stuff.

The downsides of cache-first serving policy, I guess. :goat:

(issue authored by paul. thanks sam)