skipWaiting() should return a promise

[alastaircoote]

In the course of writing the README for the project, I found out that ServiceWorkerGlobalScope.skipWaiting() actually returns a promise... that returns immediately with a value of undefined:

https://developer.mozilla.org/en-US/docs/Web/API/ServiceWorkerGlobalScope/skipWaiting

Our version just returns undefined. Should be a pretty simple fix.