Remove buildout.

I think we can remove the virtualenv management from the Makefile: let the developer choose the way he setups his Python environment. So he can use virtualenv, virtualenvwrapper or anything else... then the Makefile just supposes `pip` exists and the Python version is a supported one.

See also https://github.com/diecutter/diecutter/pull/90/files or https://github.com/novapost/django-ticketoffice/blob/master/Makefile