Add support for MQTT 3.1.1

In #90, support for MQTT 3.1.1 has been added a few months ago. Since the implementation has not received any negative feedback, it seems to work as expected and is hereby released.

There MQTT 3.1.1 protocol can be used by passing 3.1.1 as fourth parameter to the constructor, e.g. using the provided constant:

$mqtt = new \PhpMqtt\Client\MqttClient($server, $port, $clientId, \PhpMqtt\Client\MqttClient::MQTT_3_1_1);