title: Textract fails on pdftoppm error
body: <p>Running textract under python3 in OSX, a simple test programme fails with:</p>
  
  <p>textract.exceptions.ShellError: The command <code>pdftoppm page000.pdf /var/folders/hl/vxz79b3d7qd_ktk3dxs6gt680000gn/T/tmp2k0erixi/conv</code> failed because the executable
  <code>pdftoppm</code> is not installed on your system.</p>
  
  <p>But the installation of textract indicates that all the required dependencies are in place.</p>
  
  <p>Must be something silly ..... help please! </p>