capybara_fonts.rb

require 'rspec'
require 'capybara/rspec'
require 'capybara-webkit'
require 'headless'

Headless.new.start
Capybara.default_driver = :webkit

feature do
  it do 
    visit 'http://fortawesome.github.com/Font-Awesome/'
    page.save_screenshot 'screenshot.png'
  end
end

You probably need to add the domain to capybara-webkit's whitelist. Before line 11, add the following:

page.driver.allow_url('fortawesome.github.com')

This would also work:

page.driver.allow_url('github.com')

As would:

page.driver.allow_url('fortawesome.github.com/Font-Awesome/*')