Vendor virtualenv, update to 15.1.0.

bootstrap/virtualenv/CONTRIBUTING.rst

-virtualenv
-==========
-
-See docs/index.rst for user documentation.
-
-Contributor notes
------------------
-
-* virtualenv is designed to work on python 2 and 3 with a single code base.
-  Use Python 3 print-function syntax, and always ``use sys.exc_info()[1]``
-  inside the ``except`` block to get at exception objects.
-
-* virtualenv uses git-flow_ to `coordinate development`_. The latest stable
-  version should exist on the *master* branch, and new work should be
-  integrated to *develop*.
-
-* All changes to files inside virtualenv_embedded should be integrated to
-  ``virtualenv.py`` with ``bin/rebuild-script.py``.
-
-.. _git-flow: https://github.com/nvie/gitflow
-.. _coordinate development: http://nvie.com/posts/a-successful-git-branching-model/