Update min tox version to 2.0

The commands used by constraints need at least tox 2.0.
Update to reflect reality, which should help with local running of
constraints targets.

Change-Id: I343da220ef68a56448042046bf644b964ef71f62
diff --git a/tox.ini b/tox.ini
index ea9abf1..bc829d2 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,5 +1,5 @@
 [tox]
-minversion = 1.6
+minversion = 2.0
 envlist = pep8,py35,py27
 skipsdist = True