attempt to get caching+ccache working on osx - refs https://blog.travis-ci.com/2016-05-03-caches-are-coming-to-everyone

.travis.yml

@@ -1,4 +1,4 @@
-language: c
+language: generic
 
 git:
   depth: 10
@@ -39,10 +39,10 @@ matrix:
           sources: [ 'ubuntu-toolchain-r-test', 'llvm-toolchain-precise-3.5' ]
           packages: [ 'clang-3.5', 'llvm-3.5-dev' ]
     - os: osx
-      compiler: clang
+      compiler: ": clang-osx"
       # https://docs.travis-ci.com/user/languages/objective-c/#Supported-OS-X-iOS-SDK-versions
       osx_image: xcode7.3 # upgrades clang from 6 -> 7
-      env: JOBS=4 MASON_PUBLISH=true
+      env: JOBS=4 MASON_PUBLISH=true CXX="ccache clang++ -Qunused-arguments"
 
 before_install:
  - source scripts/travis-common.sh