Re-enable python2.6

I am re-enabling python2.6 in the paver script.
As far as I can see python2.7 was made mandatory just to use the ZipFile class with the "with" syntax.
We do not have the luxury to upgrade to python2.7 in one of our server (mod_wsgi is compiled vs python2.6 and we have other apps running with that version).
I have tested installation process from scratch on both env (2.6 and 2.7) and tests are running successfully.