Update Node.js versions
Update Node.js versions

Only Node.js 8, 10, and 12 are currently supported upstream. Older
versions have reached EOL.

Node.js 12 tests appear to fail, but might be fixed by
https://github.com/openvenues/node-postal/pull/21.

Until then Node.js 12 is listed as an `allowed_failure`

openvenues/node-postal,openvenues/node-postal,openvenues/node-postal