reset ccache statistics at install

.travis.yml

@@ -10,6 +10,8 @@ install:
 - if [ "$CXX" = "clang++" ] && [ "$TRAVIS_OS_NAME" = "linux" ]; then export CXX="clang++-3.7" CC="clang-3.7"; ln -sf /usr/bin/ccache /$HOME/bin/$CXX; ln -sf /usr/bin/ccache /$HOME/bin/$CC; 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
+# reset ccache statistics
+- ccache --zero-stats
 - echo ${PATH}
 - echo ${CXX}
 - ${CXX} --version