Update virtualenv to 1.11.6

1.7 doesn't appear to play nicely with the version of Python installed
on Ubuntu's trusty.

We are using this build pack with `dokku` which uses Ubuntu trusty as the base. With the vendored version of `virtualenv` I was getting errors like `ImportError: No module named _sysconfigdata_nd` when it was attempting to create the virtualenv to install hg/bzr. I believe this is due to https://bugs.launchpad.net/ubuntu/+source/python2.7/+bug/1115466, but upgrading virtualenv appears to have fixed the issue.