Update minimum tox version to 1.8
Update minimum tox version to 1.8

Other projects have updated to 1.8 or later for the minimum
tox version (nova, neutron), so we should probably do the
same.

Change-Id: Iad3c452a3e5935894e0fd9748bcb55fde5fcd04f

openstack/os-brick,openstack/os-brick