Revert "Ensure Travis uses the latest version of xctool."

This reverts commit 38fdea9,
which workaround is no longer necessary now that Travis has xctool 0.1.16 installed
(travis-ci/travis-ci#2345).

.travis.yml

@@ -6,9 +6,7 @@ language: objective-c
 
 install:
     # - `xctool`, required to build the unit test and documentation targets
-    #   Workaround for https://github.com/travis-ci/travis-ci/issues/2051#issuecomment-40817341
-    - brew update
-    - brew upgrade xctool
+    #   nothing to do here because it's already installed on Travis
 
     # - `appledoc`, used to build documentation
     #   Force the usage of appledoc 2.1 until https://github.com/inkling/Subliminal/issues/71 is addressed;