Safari: expired certificate resulted in a SW error 

The amp.dev certificate expired, which got reported in Safari as a service worker error:

<img width="710" alt="Screen Shot 2019-09-20 at 01 42 10" src="https://user-images.githubusercontent.com/380472/65347574-937acf80-dbdf-11e9-9e10-1ad755930f7a.png">