Open Xcode and, from the menu XCode -> Preferences update your Command Line Tools (Xcode 8.0).
Then do:
bundle config build.nokogiri --use-system-libraries=true --with-xml2-include="$(xcrun --show-sdk-path)"/usr/include/libxml2
bundle install
or just:
gem install nokogiri -v 1.6.8.1 -- --use-system-libraries=true --with-xml2-include="$(xcrun --show-sdk-path)"/usr/include/libxml2