p: really disable using pdiffs

pdiffs cost a lot of cpu in order to save some bandwidth
that's useless with a local mirror or proxy

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p-r: switch from python-rpy to python-rpy2

python-rpy is obsolete and planned to be removed

use a noninteractive png grDevice to avoid requiring X access

based on an initial patch from Dave Steele

Signed-off-by: Andreas Beckmann <anbe@debian.org>