certificate PDF: fix so that it's on one page, write entire date line vi...

...a text method in Prawn

- use updated PNG from Jason
- fix the generated PDF so that it's on one page.  (The auto placement of the image was pretty far off and caused the PDF to balloon into 3 pages instead.)
- rather than just paste the date on to the certificate PDF, write the entire final line to the certificate PDF via the "text" method.

TODO(?):
- update Prawn gem, make sure PDF still prints nicely
- handle long full-names properly (maybe with linewrap?)