fix travis

.travis.yml

@@ -31,9 +31,9 @@ addons:
 before_install:
 - git submodule update --init --recursive
 # - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then sudo ln -s /usr/bin/gcc-6 /usr/local/bin/gcc; sudo ln -s /usr/bin/g++-6 /usr/local/bin/g++; fi
-- brew update
-- brew install freetype libevent sqlite dylibbundler;
 - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then 
+  brew update;
+  brew install freetype libevent sqlite dylibbundler;
   sudo rm -rf /usr/local/include/unistring;
   sudo rm -rf /usr/local/include/lzma;
   sudo rm -f /usr/local/include/CGAL/stddef.h; fi