LoadError: cannot load such file — nokogiri

I am trying to push my Ruby on Rails project to DigitalOcean by using Capistrano.

I am following this guide here: https://www.digitalocean.com/community/tutorials/deploying-a-rails-app-on-ubuntu-14-04-with-capistrano-nginx-and-puma

I followed every steps, and everything was fine until I reached step 7 which I run:

cap production deploy:initial

on my local machine. This command gave me this error:

http://i.stack.imgur.com/EahwV.png

I never require nokogiri in my gemfile or install nokogiri for my app, so it is very wierd that this error appear. So, somebody help me please? I have stuck on this for a few days already :(

Thanks!