Pin down Pillow to 1.x for python 2.4 and 2.5.

Pillow 2.x supports python >= 2.6, for python <=2.5 the Pillow 1.x series needs to be used.

I had to create python version specific `pil-install-args` variables because the default `pil-install-args` is overridden by later loaded python versions (see base.cfg)..