small changes to make this work on Ubuntu 12.04 called via symlink

I added a resolution of the script's real path using "readlink -f" to find its actual location even if called via symlink (e.g. in "~/bin/OCRmyPDF").

Further the minimal required "parallel" version was set to "20130222" for the use of the quote parameter "-q". On Ubuntu 12.04 I have version "20121122" which has the "-q" parameter and works just fine, so I lowered the version requirement.