Update to tox.ini

When do the action "tox -e pep8", result is "ValueError: No closing
quotation". Which is caused by the tox.ini. The min tox version is
1.6. In my environment, tox version is 2.2.1. If the tox version is
more than 2.3, the error will not happen.

Depends-on:  Iee7b043ac7d381dadf89d26098f69e935ed81d6b
Change-Id: Id10ddf6244e7e25e6f66c97773d426b0b4789479

Author: yangweiwei

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 1.6
+minversion = 2.3
 envlist = py35,py27,pep8
 skipdist = True