virtualenv: upgraded package to upstream

Upgraded dev-python/virtualenv to version 13.1.2 on amd64

BUG=chrome-os-partner:51877
TEST=emerge virtualenv

Change-Id: I427440fd84867e97523a1fe592bf3f93978b6a46
Reviewed-on: https://chromium-review.googlesource.com/336600
Commit-Ready: Hung-Te Lin <hungte@chromium.org>
Tested-by: Hung-Te Lin <hungte@chromium.org>
Reviewed-by: Hung-Te Lin <hungte@chromium.org>
Reviewed-by: Wei-Ning Huang <wnhuang@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
6 files changed