Tox fixed the openstack breaking bug use latest

Hurray upstream tox merged a fix for the issue that had us stuck on
1.6.1. They have now released a new version so we can consume that fix.
Start using latest tox again to pull that in.

See https://bitbucket.org/hpk42/tox/issue/150/posargs-configerror
for all the details.

Change-Id: Ia2c7cb07c98c9942113309054b4c40af23596d04
Fixes-bug: 1274135
Merge "Tox fixed the openstack breaking bug use latest"