tox: Add travis config for 3.8-dev
tox: Add travis config for 3.8-dev

As reported in https://github.com/tox-dev/tox-travis/issues/147, the
python factor in tox-travis needs to match the Travis python environment
and not the python version. Therefore, add a separate line.

linux-system-roles/network,linux-system-roles/network,linux-system-roles/network