title: Bundler error: builtin_gem? when deploying on Heroku
body: <p>Following this tutorial: <a href="https://devcenter.heroku.com/articles/getting-started-with-rails4" rel="nofollow">https://devcenter.heroku.com/articles/getting-started-with-rails4</a></p>
  
  <p>When I attempt <code>git push heroku master</code> I encounter the following error:</p>
  
  <blockquote>
    <p>/tmp/build_50f24046-6d63-4f42-8238-f4cc4b983d77/vendor/bundle/ruby/1.9.1/gems/bundler-1.6.1/lib/bundler/source/rubygems.rb:299:in <code>builtin_gem?': undefined method</code>include?' for nil:NilClass (NoMethodError)</p>
  </blockquote>