Downloaded from https://pypi.python.org/packages/source/v/virtualenv/virtualenv-14.0.1.tar.gz
and extracted without any modifications.
This appears to speed up `mach configure` for artifact builds by ~3.3s on
my Linux machine (~16.3s to 13.0s). A significant part of this appears
to be newer version of pip caching and reusing wheels after first
install.
This should be a rubber stamp review, as all changes are from trusted
upstream packages.
