Nokogumbo has been merged into Nokogiri, so an explicit require for nokogumbo is no longer required in the Nokogumbo adapter.

See sparklemotion/nokogiri#2204 for details.

In my project, I removed the Nokogumbo gem from the Gemfile and got errors about not being able to find nokogumbo.rb. Adding an empty nokogumbo.rb file did the trick, but perhaps the explicit require should be removed from the adapter file?