Add missing double quotes to readlink call. Opening paths with spaces now works.

I was trying to make it possible to double click on .pde files on Ubuntu 13.04 following http://forum.processing.org/topic/ubuntu-12-10-launch-pde-files-from-nautilus
I noticed that files found in paths that include spaces would not open. I fixed it by adding double quotes to a readlink call at the end of the Processing launcher script. Other similar calls to readlink in the same file already included double quotes.