install/upgrade xctool from HEAD

since Xcode 7 GM support is not officially released on Homebrew—yet.

facebookarchive/xctool#528

Author: bgerstle

Makefile

@@ -26,6 +26,7 @@ travis-get-deps: ##Install dependencies for building on Travis
 travis-get-deps: xcode-cltools-check submodules
 	@brew update; \
 	brew install carthage || brew upgrade carthage; \
+	brew install xctool --HEAD || brew install xctool --HEAD; \
 	bundle install --without dev;
 
 #!!!!!