For CI - switch off the OSX precache build.

It will be reimplemented as build stage later when Travis supports conditional build stage.

.travis.yml

@@ -487,7 +487,7 @@ after_script: rake ci_teardown_cache
 
 ---
 
-branch: {name: OSX-CI, active: yes, mandatory: yes}
+branch: {name: OSX-CI, active: yes}
 language: objective-c
 cache: {directories: $HOME/.ccache}
 osx_image: xcode8.3