[TravisCI] use node 8 on Mac

.travis.yml

@@ -97,6 +97,7 @@ install_linux: &install_linux
   - export HAXE_STD_PATH="$TRAVIS_BUILD_DIR/std"
 
 install_osx: &install_osx
+  - (nvm ls 8 || nvm install 8) && nvm use 8
   # Install dependencies
   - travis_retry brew update --merge
   - brew uninstall --force brew-cask # https://github.com/caskroom/homebrew-cask/pull/15381