Relax min tox from 1.6 to 1.4

Fedora 20 only ships with tox 1.4.2 and this appears to work
without trouble. So relax min tox from 1.6 to 1.4 on the basis
that there's no immediately obvious need for the 1.6.x series.

Change-Id: I673936fa5ca56f7c9641a90d89f43fc72eb16c19