Assert Xcode version when building fbsimctl
Assert Xcode version when building fbsimctl

Summary:
Provide better error message if somebody installs via Homebrew and does not have Xcode 8 installed.
Closes https://github.com/facebook/FBSimulatorControl/pull/323

Differential Revision: D4029141

Pulled By: lawrencelomax

fbshipit-source-id: fa719792c5fbb8e52ed67bb8d45a540f66f2d061

plu/FBSimulatorControl,facebook/FBSimulatorControl,StevenUpForever/FBSimulatorControl,facebook/FBSimulatorControl,StevenUpForever/FBSimulatorControl,plu/FBSimulatorControl,facebook/FBSimulatorControl,plu/FBSimulatorControl,StevenUpForever/FBSimulatorControl,plu/FBSimulatorControl,facebook/FBSimulatorControl,StevenUpForever/FBSimulatorControl,ichu501/FBSimulatorControl,facebook/FBSimulatorControl