server DDP: Use low-level websocket-driver module

We want to support running DDP through a corporate proxy, but the
higher-level faye-websocket can't support that and won't be changed to
allow that: https://github.com/faye/faye-websocket-node/pull/30

Fair enough. Let's just switch to the lower-level module, since we don't
care about getting a browser-compatible websocket API.

This is a first step towards fixing #2515.