travis: disable unsupported versions
travis: disable unsupported versions

Virtualenv with `--system-site-packages` is only supported with the
official Ubuntu Python versions.

Signed-off-by: noxdafox <43c758619f0ced9dc5c51a50803f3466c86e4c3d@gmail.com>

noxdafox/see,F-Secure/see