docs(install): switch from rnpm to react-native
docs(install): switch from rnpm to react-native

RNPM is merged into React Native core - see [here](https://github.com/rnpm/rnpm/blob/master/README.md#dear-friends)
bsudekum/react-native-mapbox-gl,bsudekum/react-native-mapbox-gl,bsudekum/react-native-mapbox-gl,bsudekum/react-native-mapbox-gl