TRAVIS: Use CCache for OSX Builder

The OSX builder keeps failing with timeouts and thus notifying false
positives of build errors.

This was not using ccache to speed up the build, so am trying to
implement usage of this.

.travis.yml

@@ -66,9 +66,12 @@ matrix:
         - sudo ln -s $(which ccache) /usr/lib/ccache/clang++
     - os: osx
       compiler: clang
-      cache:
+      cache: ccache
         directories:
           - $HOME/Library/Caches/Homebrew
+      before_script:
+        - sudo ln -s $(which ccache) /usr/lib/ccache/clang
+        - sudo ln -s $(which ccache) /usr/lib/ccache/clang++
       before_cache:
         - brew cleanup