Roll virtualenv 12.0->15.0.2

bootstrap/virtualenv/tox.ini

+[tox]
+# env names must be a valid python  binary name, unless they have a
+# separate configuration
+envlist =
+    python{2.6,2.7,3.3,3.4,3.5}, pypy{,3}, crosspython{2,3}, docs
+
+[testenv]
+deps =
+    mock
+    pytest
+commands =
+    py.test
+
+# Creating a python3 venv with a python2-based virtualenv
+[testenv:crosspython2]
+basepython=python2
+deps = 
+commands =
+    virtualenv -p python3 {envtmpdir}/{envname}
+    {envtmpdir}/{envname}/bin/python -V 2>&1 | grep "Python 3"
+
+# Creating a python2 venv with a python3-based virtualenv
+[testenv:crosspython3]
+basepython=python3
+deps = 
+commands =
+    virtualenv -p python2 {envtmpdir}/{envname}
+    {envtmpdir}/{envname}/bin/python -V 2>&1 | grep "Python 2"
+
+[testenv:docs]
+deps =
+    sphinx
+    readme
+commands =
+    sphinx-build -W -b html -d {envtmpdir}/doctrees docs docs/_build/html
+    python setup.py check -m -r -s