Update Travis CI config to use the Xcode 8.1 image.
Update Travis CI config to use the Xcode 8.1 image.

ohwutup/ReactiveCocoa,calebd/ReactiveCocoa,ohwutup/ReactiveCocoa,walkingsmarts/ReactiveCocoa,walkingsmarts/ReactiveCocoa,walkingsmarts/ReactiveCocoa,ohwutup/ReactiveCocoa,calebd/ReactiveCocoa,calebd/ReactiveCocoa