Switch ObjC Travis testing to using xctool
Switch ObjC Travis testing to using xctool

xctool is preinstalled on the Travis OS X images and it seems to do better with
iOS simulator flake, so use it instead of xcodebuild.

xctool also is less chatty compared to xcodebuild, so it makes the logs a little
easier to read.

google/protobuf,Livefyre/protobuf,google/protobuf,google/protobuf,Livefyre/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,Livefyre/protobuf,Livefyre/protobuf,google/protobuf,Livefyre/protobuf,google/protobuf