When initially deploying you should not have to install prerequisites to use Ruby but let the automatic deployment take care of this. This gem let's you install Ruby from scratch using Rbenv under the hood.

Add these gems to your application's Gemfile:

gem 'capistrano', '~> 3.6'
gem 'capistrano-rbenv', '~> 2.0'
gem 'capistrano-ubuntu-ruby-setup'

And then execute:

$ bundle

Setup your deploy.rb according to capistrano-rbenv and add the following requires to your Capfile:

require 'capistrano/rbenv'
require 'capistrano/rbenv_install'
require 'sshkit/sudo'
require 'capistrano/ubuntu_ruby_setup'

Run bundle exec cap <environment> deploy and profit.

The gem is available as open source under the terms of the MIT License.