Update travis to Xcode 9.1 (#427)
Update travis to Xcode 9.1 (#427)

* Update travis to Xcode 9.1
* Prepend pod lib lint with bundle exec

firebase/firebase-ios-sdk,firebase/firebase-ios-sdk,firebase/firebase-ios-sdk,firebase/firebase-ios-sdk,firebase/firebase-ios-sdk,firebase/firebase-ios-sdk,firebase/firebase-ios-sdk