Pillow 4.0 recently released, and it's the first version of Pillow to
drop Python 2.6 support. This means that 2.6 users who use Pillowfight
end up with a version of Pillow they can't install.

This change adds a version cap for Pillow when running under Python 2.6.
It also increases the minimum version of Pillow to the latest
2.6-supported version.

Tested "building" the pillowfight package on 2.6 and 2.7. Verified the

correct versions of Pillow were installed.