* drop support for Node.js v4; closes #7753 - remove version 4 from Travis build matrix - update environment support doc - update `CONTRIBUTING.md` - update `engines` field of root `package.json` Signed-off-by: Christopher Hiller <boneskull@boneskull.com> * update engines for babylon Signed-off-by: Christopher Hiller <boneskull@boneskull.com>