Travis: Fix OS X build

Probably due to additional Travis caching, cmake is now already
installed when the script runs. This causes the unlink to remove the
symlink to the executable, which is then not re-added by the install
(since it's already installed).

.travis-deps.sh

@@ -28,7 +28,6 @@ if [ "$TRAVIS_OS_NAME" = "linux" -o -z "$TRAVIS_OS_NAME" ]; then
     fi
 
 elif [ "$TRAVIS_OS_NAME" = "osx" ]; then
-    brew update > /dev/null # silence the very verbose output
+    brew update
-    brew unlink cmake || true
     brew install cmake qt5 sdl2 dylibbundler
 fi