Disable homebrew auto-update

.travis.yml

@@ -38,6 +38,8 @@ matrix:
       env: FUZZER=true
     - os: osx
       compiler: clang
+      # disable homebrew auto update which takes a lot of time
+      env: HOMEBREW_NO_AUTO_UPDATE=1
       cache:
         directories:
           - $HOME/Library/Caches/Homebrew