[uievents] Use https://rawgit.com URL for Keyboard Event Viewer (#72)

The change to `beforeinput` is not reflected in https://cdn.rawgit.com/w3c/uievents/gh-pages/tools/key-event-viewer.html

According to [Rawgit](https://rawgit.com/) `https://cdn.rawgit.com` should be used for production
> It's best to use a specific tag or commit hash in the URL (not a branch). Files are cached permanently after the first request.

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https://github.com/w3c/uievents/issues/72