Update the min version of tox

In Train, we will use python3.6 and 3.7 for which
the minimum tox version required is 2.5[1]

[1]https://tox.readthedocs.io/en/latest/changelog.html#v2-6-0-2017-02-04

Change-Id: I9b3dec141bd9e62739be00b69d033807cc2a2bd7

tox.ini

 [tox]
-minversion = 2.0
+minversion = 2.5.0
 skipsdist = True
 envlist = py36,py27,pep8,pypy