Update readme to remove beta.15 note
Update readme to remove beta.15 note

FirebaseExtended/emberfire,MattMSumner/emberfire,FirebaseExtended/emberfire,leifcr/emberfire,rtablada/emberfire,rtablada/emberfire,MattMSumner/emberfire,FirebaseExtended/emberfire,FirebaseExtended/emberfire,leifcr/emberfire,IsaiahJTurner/emberfire,IsaiahJTurner/emberfire