update TravisCI to run yarn in node 8 (drop node 6)
update TravisCI to run yarn in node 8 (drop node 6)

oVirt/ovirt-web-ui,oVirt/ovirt-web-ui,mareklibra/userportal,mareklibra/userportal,mareklibra/userportal,mareklibra/userportal,oVirt/ovirt-web-ui