Update "webvr" URLs (#480)
Update "webvr" URLs (#480)

This change makes https://immersive-web.github.io/webvr/spec/1.1/ the
URL for the WebVR spec (key: "webvr"), rather than the URL
https://w3c.github.io/webvr/spec/latest/ (which now redirects to a 404).
The change also updates the "repository" URL to
https://github.com/immersive-web/webxr (which is where the old
https://github.com/w3c/webvr now redirects to).
tobie/specref,tobie/specref