Update Vagrant key location + follow redirects

It seems that Github changed location of raw files (I assume it's now under CDN as the rest of the static content).
The `curl` doesn't follow the new location by default if it gets `3xx` response, so the key will be missing in the box.

I've updated the URL and added [-L option](http://curl.haxx.se/docs/manpage.html#-L) which will prevent these issues in the future.