Not compatible with dompdf 0.7 or 0.8

As of dompdf 0.7.0, there is no longer a dompdf_config_inc.php file, so 7.x-2.0 fails straight away.

Either update code or specify the compatible dompdf versions in the documentation.

I was attempting to update dompdf due to the error documented here: https://www.drupal.org/node/2188717