Allow reactphp 0.4.* and evenment 2.0 to be used

For a project I'm using react 0.4 everywhere and I would like to add [DNode](https://github.com/WyriHaximus/RatchetCommands/issues/7) support to it. Currently composer.json prevents this. With this proposed change the eventment 2.0 and react 0.4 versions can be used.