Run validate-modules with python 2 only.

This allows validate-modules to run in an environment where
python 3 is the default. This will no longer be necessary once
validate-modules is updated to work with both python 2 and 3.

test/sanity/validate-modules/validate-modules

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python2
 # -*- coding: utf-8 -*-
 #
 # Copyright (C) 2015 Matt Martz <matt@sivel.net>