add deprecation for MapBox

I've added a `deprecated_in_favor_of` in order to shepherd folks over to the `Mapbox-iOS-SDK` pod instead. 

Should I be doing this for all of the older, pre-`1.1.0` versions as well, or just this latest one? 

Also, the new `Mapbox-iOS-SDK` picks up at `1.2.0`, which `MapBox` doesn't have.