try to fix osX build issue

.travis.yml

@@ -17,6 +17,7 @@ compiler:
 
 script:
   - if [[ "${TRAVIS_OS_NAME}" == "osx" ]] && [[ "${CC}" == "gcc" ]]; then
+        brew update;
         brew install gcc48;
         export CC=gcc-4.8;
         export CXX=g++-4.8;