Force python 2.6 on install (compatibility with 2.7 is an issue in virtualenv)

[jkliff]

Please consider the following fix for the issue described below:

On a clean clone, ./install fails as follows.

% ./install.sh installing theory prerequisites and Python virtual environment to /home/john/Desktop/src/theory//env/ New python executable in /home/john/Desktop/src/theory//env/bin/python Traceback (most recent call last): File "/home/john/Desktop/src/theory//env/lib/python2.7/site.py", line 67, in import os File "/home/john/Desktop/src/theory//env/lib/python2.7/os.py", line 398, in import UserDict File "/home/john/Desktop/src/theory//env/lib/python2.7/UserDict.py", line 83, in import _abcoll File "/home/john/Desktop/src/theory//env/lib/python2.7/_abcoll.py", line 11, in from abc import ABCMeta, abstractmethod File "/home/john/Desktop/src/theory//env/lib/python2.7/abc.py", line 8, in from _weakrefset import WeakSet ImportError: No module named _weakrefset ERROR: The executable /home/john/Desktop/src/theory//env/bin/python is not functioning ERROR: It thinks sys.prefix is '/home/john/Desktop/src/theory' (should be '/home/john/Desktop/src/theory/env') ERROR: virtualenv is not compatible with this system or executable

Narrowing it down, the issue lies in a bug on virtual env (https://bugs.launchpad.net/ubuntu/+source/python-virtualenv/+bug/662611) in which runnig it from py2.7 is the culprit. Case in point, on my system python 2.7 is indeed the default one.

The issue is easily solved forcing the python which executes go-pylons to be 2.6.

Probably this should check more resiliently for the version of the python executable on different environments, but as far as it goes with typical aliasing of python versions (python2.5, python2.6, etc), this works properly.