Disable index-while-compiling in Travis (#2452)
Disable index-while-compiling in Travis (#2452)

We don't need to use Xcode's index-while-compiling during builds. Folks have
seen ~8% speedup with builds here.

Seen at https://twitter.com/steipete/status/1100404024224804871
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