Add Socket.io v2 support

[marshallswain]

Socket.io 2 was recently released.

https://socket.io/

[eddyystop]

Socket.IO 2.0.0
13 May 2017 – Damien Arrachequesne (@darrachequesne)
Socket.IO 2.0.0 is finally here! This major release brings several performance improvements:

• uws is now the default Websocket engine. It should bring significant improvement in performance (particularly in terms of memory consumption) (release note of Engine.IO)
• the Engine.IO and Socket.IO handshake packets were merged, reducing the number of roundtrips necessary to establish a connection. (#2833)
• it is now possible to provide a custom parser according to the needs of your application (#2829). Please take a look at the example for more information.

Please note that this release is not backward-compatible, due to:

• a breaking change related to utf-8 encoding in engine.io-parser
• an update to make the socket id on the client match the id on the server-side (#1058)
• Related packages were updated accordingly:

socket.io-redis version 5.x
socket.io-emitter version 3.x

[daffl]

This has already been done in #75 and released as v2.0.0 of feathers-socketio.