xctool build is broken; use xcodebuild
xctool build is broken; use xcodebuild

Travis uses xctool by default to build and test the project, but
this is currently broken for xcode 9:

https://github.com/facebook/xctool/issues/738

Switch to xcodebuild for now.

WilliamDenniss/AppAuth-iOS,openid/AppAuth-iOS,openid/AppAuth-iOS,WilliamDenniss/AppAuth-iOS,WilliamDenniss/AppAuth-iOS,openid/AppAuth-iOS