Limit the version of Pygments on Python versions prior to 2.6.

Pygments 2.0 dropped support for Python 2.4 and 2.5. If we're running
on such a version, we need to ensure we're not going to pull down
Pygmnets 2.0.

This updates the setup.py to handle this version requirement.

Reviewed at https://reviews.reviewboard.org/r/6692/