chore(node): use node 6 as it required in our package.json

Created by: Johann-S

it does not fix all of our build issues on this v3 but it helps 😆

fixed: https://travis-ci.org/twbs/bootstrap/jobs/375776929#L1668 new build issue: https://travis-ci.org/twbs/bootstrap/jobs/375780714#L1540

/CC @mdo, @XhmikosR