install ccache on travis osx build slave

.travis.yml

@@ -8,6 +8,8 @@ install:
 - if [ "$CXX" = "g++" ]; then export CXX="g++-4.9" CC="gcc-4.9"; fi
 # /usr/bin/clang is 3.4, lets override with modern one.
 - if [ "$CXX" = "clang++" ] && [ "$TRAVIS_OS_NAME" = "linux" ]; then export CXX="clang++-3.7" CC="clang-3.7"; fi
+# ccache on OS X needs installation first
+- if [ "$TRAVIS_OS_NAME" = "osx" ]; then brew install ccache; export PATH="/usr/local/opt/ccache/libexec:$PATH"; fi
 - echo ${PATH}
 - echo ${CXX}
 - ${CXX} --version