Change `Elephant.io` url. [skip ci]

Change `Elephant.io` url to the fork as the original repo is no longer maintained.

README.md

 * [Amp](https://github.com/amphp/amp) - An event driven non-blocking I/O library.
 * [Broadway](https://github.com/broadway/broadway) - An event source and CQRS library.
 * [CakePHP Event](https://github.com/cakephp/event) - An event dispatcher library.
-* [Elephant.io](https://github.com/Wisembly/Elephant.io) - Yet another web socket library.
+* [Elephant.io](https://github.com/ElephantIO/elephant.io) - Yet another web socket library.
 * [Evenement](https://github.com/igorw/evenement) - An event dispatcher library.
 * [Event](https://github.com/thephpleague/event) - An event library with a focus on domain events.
 * [Fast CGI Client](https://github.com/hollodotme/fast-cgi-client) - A client to make synchronous/asynchronous requests through php-fpm socket.