Install xctool from master

Attempt to fix test timeouts with most recent commits on xctool's master branch

Author: JrGoodle

.travis.yml

@@ -1,6 +1,9 @@
 osx_image: xcode7
 language: objective-c
-install: script/bootstrap
+before_install: brew update
+install:
+  - brew uninstall xctool && brew install --HEAD xctool
+  - script/bootstrap
 script: script/cibuild
 notifications:
   email: false