title: Prawn doesn't draw a horizontal rule
body: <p>I want to draw a simple horizontal rule. What I'm doing is:</p>
  
  <pre><code>move_down 30
  horizontal_rule
  </code></pre>
  
  <p>and Gemfile</p>
  
  <pre><code>gem 'prawn', :git =&gt; "https://github.com/prawnpdf/prawn.git", branch: 'master' 
  </code></pre>
  
  <p>It doesn't draw anything.</p>