Simplify tests by supporting new Socket API

[SimonFrings]

This changeset simplifies tests by supporting the new Socket API with the new SocketServer.

// old (still supported)
$server = new React\Socket\Server(0);

// new
$server = new React\Socket\SocketServer('127.0.0.1:0');

Note that this doesn't affect the API of this package at all, but does use the improved API of the referenced Socket component. Existing code continues to work as is.

Together with #56, we can now fully rely on the default loop and no longer need to skip any arguments with null values.

Builds on top of reactphp/socket#264 and reactphp/socket#263

[clue]

@SimonFrings Thanks for the update, changes LGTM!