Work around bug#77932 in pdftohtml

Sometimes pdftohtml will generate thousands of images when converting an
image embedded in a PDF. This causes a request spike when a user tries
to view the converted PDF as HTML.

See https://bugs.freedesktop.org/show_bug.cgi?id=77932 for the bug
report.
Merge branch 'issues/772-pdf-image-conversion' into rails-3-develop