upgrade node v6.9.1 in travis yml
upgrade node v6.9.1 in travis yml

testshallpass/react-native-simple-stepper,devBrian/react-native-simple-stepper,devBrian/react-native-simple-stepper,testshallpass/react-native-simple-stepper,testshallpass/react-native-simple-stepper,devBrian/react-native-simple-stepper