docs: update the list of supported Node.js versions

The server uses `timeout.refresh()` (see [1]), which was added in Node.js 10.2.0.

Reference: https://nodejs.org/api/timers.html#timeoutrefresh

Related: https://github.com/socketio/engine.io/issues/686

[1]: https://github.com/socketio/engine.io/commit/37474c7e67be7c5f25f9ca2d4ea99f3a256bd2de