I have done all kinds of research and tried many different things. I know this question has been answered many times, but none of the suggested solutions are working for me.
If you installed Nokogiri with gem install nokogiri, you can resolve this warning by running gem pristine nokogiri to recompile the gem's C extension.
If you installed Nokogiri with bundle install, you can resolve this warning by running bundle exec gem pristine nokogiri to recompile the C extension of the gem wherever Bundler installed it.