Remove the hard-coded version number from the PDF documentation link

Use the /latest/*manual-pdf link for the OMERO and Bio-Formats instead. This
allows a patch release such as 4.4.8.1 to keep pointing at the 4.4.8 PDF
documentation (assuming the redirected links are not updated).
Also remove the PDF_URL replacement key in gen.py and bfgen.py
Merge pull request #20 from sbesson/pdf_manual_link

Remove the hard-coded version number from the PDF documentation link