title: RAILS_ROOT not longer valid when loading images with the prawnto_2 gem
body: <p>I'm in the process of upgrading my app from Rails 3.0 to Rails 3.1.</p>
  
  <p>I've binned my old plugins in favour of gems where possible and that includes replacing the old prawnto plugin with this nice shiny new <a href="https://github.com/forrest/prawnto" rel="noreferrer">prawnto_2</a> gem.</p>
  
  <p>Most things seem fine, but I can't get images to load as before. The code in my PDF view is </p>
  
  <pre><code>pdf.image open("#{RAILS_ROOT}/public/images/logo.png")
  </code></pre>
  
  <p>but I get the following error</p>
  
  <pre><code>uninitialized constant ActionView::CompiledTemplates::RAILS_ROOT
  </code></pre>
  
  <p>I realise that the location of the image file will change as I'm using assets and the image is no longer stored in the public folder.</p>