* 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>