fix: install instruction
fix: install instruction

rnpm install is 'save' per default, additionally adding the flag causes the following error:
error: unknown option `--save'
SudoPlz/react-native-bugsnag,SudoPlz/react-native-bugsnag,SudoPlz/react-native-bugsnag,SudoPlz/react-native-bugsnag,SudoPlz/react-native-bugsnag,SudoPlz/react-native-bugsnag,SudoPlz/react-native-bugsnag,SudoPlz/react-native-bugsnag