tox miniversion update to 3.1.1

In 3.1.1, tox had fixed:

mishandling the basepython directive [1]
ignoring PYTHONDONTWRITEBYTECODE [2]
More details in [3]

[1] tox-dev/tox#477
[2] tox-dev/tox@336f4f6
[3] http://lists.openstack.org/pipermail/openstack-dev/2018-July/thread.html#132075

Change-Id: I37ab7fd16ca3125870a1201fb3b2e86c61564f47
diff --git a/tox.ini b/tox.ini
index 8e6b2bf..153ebbf 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,5 +1,5 @@
 [tox]
-minversion = 2.0
+minversion = 3.1.1
 envlist = py3,pypy,pep8
 skipsdist = True