Turn off bottles for xctool

xctool currently needs to be compiled with the Xcode version you intend to run it with. Because of bottles this may not be the case.

This is a stopgap to unblock some people. It is still not correct, as you could use a different Xcode than the one used to compile during Homebrew install, install a later version after you compile via Homebrew, etc.

Eventually we're going to either have xctool handle both or have two separate Homebrew formulas so people can pick and choose.

Details @ https://github.com/facebook/xctool/issues/380#issuecomment-56143178