Data retrieval code won't work if the *beta* Europe server is chosen for Realtime DB

Hi,

The provided code won't work if european (Belgium) server is chosen for realtime db, because it does not sits on the default url. I don't know how to solve this properly, but a workaround is to modify the Firebase DB reference initiation like this:

```mFirebaseDatabaseReference = FirebaseDatabase.getInstance("url-to-europe-db").getReference();```

Maybe this should be stated in codelabs, because gave a few hours to debug :)

thanks,
adam