add an option to specify the python version of the virtualenv

Hi Mozilla!

This pull request correct this issue 
"load_virtualenv crashes when virtualenv has a different Python version #688"

Now, if you run circus with python2.7 and if your application use chaussette and a python3.3 virtualenv,
you can use the following option in the watcher configuration
"virtualenv_py_ver=3.3"

Bye

Morgan