title: ExecJS and could not find a JavaScript runtime
body: <p>I'm trying to use the <a href="https://github.com/RailsApps/rails3-mongoid-devise">Mongoid / Devise Rails 3.1 template</a> (<em><a href="http://mongoid.org/">Mongoid</a></em> and <em><a href="https://github.com/plataformatec/devise">Devise</a></em>), and I keep getting an error stating <a href="http://rubygems.org/gems/execjs">ExecJS</a> cannot find a JavaScript runtime.  Fair enough when I didn't have any installed, but I've tried installing <a href="http://en.wikipedia.org/wiki/Nodejs">Node.js</a>, <a href="https://github.com/nu7hatch/mustang">Mustang</a> and the <a href="https://github.com/cowboyd/therubyracer">Ruby Racer</a>, but nothing is working. </p>
  
  <p>I could not find a JavaScript runtime. See <a href="https://github.com/sstephenson/execjs">sstephenson/ExecJS</a> (GitHub) for a list of available runtimes (<code>ExecJS::RuntimeUnavailable</code>).</p>
  
  <p>What do I need to do to get this working?</p>