setup.py: peg jinja to version <2.7

This is needed for Python 3.2 compatibility. 2.7 works with Python 3.3,
but since we're targetting Ubuntu 12.04 LTS as our supported platform
for the time being, we're stuck with Python 3.2.
Merge pull request #1 from larsbutler/jinja-python3-compat

setup.py: peg jinja to version <2.7