[docs] Fix incorrect socket.io-protocol version in Readme (#89)

Ref: https://github.com/socketio/socket.io-protocol

Readme.md

@@ -4,7 +4,7 @@
 [![Build Status](https://secure.travis-ci.org/socketio/socket.io-parser.svg?branch=master)](http://travis-ci.org/socketio/socket.io-parser)
 [![NPM version](https://badge.fury.io/js/socket.io-parser.svg)](http://badge.fury.io/js/socket.io-parser)
 
-A socket.io encoder and decoder written in JavaScript complying with version `3`
+A socket.io encoder and decoder written in JavaScript complying with version `4`
 of [socket.io-protocol](https://github.com/socketio/socket.io-protocol).
 Used by [socket.io](https://github.com/automattic/socket.io) and
 [socket.io-client](https://github.com/automattic/socket.io-client).