Add flag to disable wheels for python 2.6

Setuptools 0.8 is required for pip wheel support, Python 2.6 and older only
support setuptools 0.7, we therefor need to disable wheels.
Fix wheel flag for pip commands

Python 2.6 does not support setuptools < 0.8 which is required for pip wheel
support, older versions need to use the --no-use-wheel flag