title: Rails 4 PDFkit installation fails
body: <p>For my site i want to convert an HTML CSS page in a PDF.
  I use the Gem "pdfkit" and follow the instructions from github and a rails cast.
  But i can't get it working on rails 4
  can someone help me?</p>
  
  <p>I get this error</p>
  
  <pre><code>command failed (exitstatus=1): /usr/local/rvm/gems/ruby-2.1.0/bin/wkhtmltopdf --quiet --page-size Letter --margin-top 0.75in --margin-right 0.75in --margin-bottom 0.75in --margin-left 0.75in --encoding UTF-8 - -
  </code></pre>
  
  <p>Gem File</p>
  
  <pre><code>gem "pdfkit"
  gem "wkhtmltopdf-binary"
  </code></pre>
  
  <p>config/application.rb</p>
  
  <pre><code>config.middleware.use "PDFKit::Middleware"
  </code></pre>