title: Runtime Error only when deployed on Heroku
body: <p>My application works well locally but when pushed to heroku, it crashes on a particular action: opening a PDF document.  I'm using <a href="https://github.com/pdfkit/pdfkit" rel="nofollow">PDFKit</a> for document rendering which uses wkhtmltopdf middleware.  Here's my error message: </p>
  
  <p><code>2015-03-20T23:35:11.306857+00:00 app[web.1]: Processing by PcsController#show as HTML
  2015-03-20T23:35:11.605847+00:00 app[web.1]:   Rendered pcs/show.html.erb within layouts/application (27.5ms)
  2015-03-20T23:35:11.614100+00:00 app[web.1]: Completed 200 OK in 307ms (Views: 45.8ms | ActiveRecord: 27.1ms)
  2015-03-20T23:35:13.127259+00:00 heroku[router]: at=info method=GET path="/favicon.ico" host=myhost.herokuapp.com request_id=671d7282-5765-475e-bdf1-411e8d66efe3 fwd="71.45.94.25" dyno=web.1 connect=1ms service=8ms status=304 bytes=111
  2015-03-20T23:35:12.997916+00:00 app[web.1]: RuntimeError (command failed (exitstatus=0): /app/vendor/bundle/ruby/2.1.0/bin/wkhtmltopdf --page-size Letter --margin-top 0.75in --margin-right 0.75in --margin-bottom 0.75in --margin-left 0.75in --encoding UTF-8 --print-media-type - -):
  </code></p>
  
  <p>Obviously, there is some issue with wkhthmltopdf, but I can't seem to locate a solution via the documentation or the internets.  These are default settings - I had once changed them using meta tags, but have since decided to revert to defaults (layout decision, not a function decision).  I'm using Unicorn as my web server and have 3 worker processes allocated.  Hoping someone has some ideas.</p>