I had to run:
sudo gem install nokogiri -- --with-xml2-include=/usr/include/libxml2 --with-xml2-lib=/usr/lib --with-xslt-include=/usr/include/libxslt --with-xslt-lib=/usr/lib on the server from the directory: /usr/lib/ruby/gems/2.1.0/gems Note: This server is managed by Moonshine. I could not get rid of this warning any other way. Reference: How to fix Nokogiri on Ubuntu 
