title: Uninitialized constant Capybara::Webkit::Configuration
body: <p>I have an issue due adding the <a href="https://github.com/oesmith/puffing-billy" rel="nofollow noreferrer">puffing-billy</a> to my capybara-webkit features. 
  I followed by instructions:</p>
  
  <p>rails_helper.rb:</p>
  
  <pre><code>require 'billy/capybara/rspec'
  </code></pre>
  
  <p>capybara.rb:</p>
  
  <pre><code>Capybara.javascript_driver = :webkit_billy
  #and other staff bellow
  </code></pre>
  
  <p>Due running feature I have:</p>
  
  <pre><code>NameError:
              uninitialized constant Capybara::Webkit::Configuration
            # /Users/ir/.rvm/gems/ruby-2.5.0@guestready/gems/puffing-billy-0.11.1/lib/billy/browsers/capybara.rb:43:in `block in register_webkit_driver'
            # /Users/ir/.rvm/gems/ruby-2.5.0@guestready/gems/capybara-2.17.0/lib/capybara/session.rb:101:in `driver'
  </code></pre>
  
  <p>What I m missing?</p>