Fix mailman buildout on Centos 7 (and older OSes) with broken virtualenv

Virtualenv has a bug in it that doesn't create venv's correctly

https://github.com/pypa/virtualenv/issues/463#issuecomment-25904873

This is fixed in upstream, but Centos 7 doesn't ship with a new enough version for this to not break, an example bug report can be found here:

https://gitlab.com/mailman/mailman-bundler/issues/19/

this basically just assumes virtualenv is broken, and upgrades it into the 2.7 virtualenv that buildout is using just prior to trying to create the 3.x (3.4 in this case) virtualenv. This is obviously a bit of a sledge hammer, but it does ensure a vaguely known state.

Signed-off-by: John 'Warthog9' Hawley warthog9@eaglescrag.net