Update the package for v1 of the MQTT client (#1)

* Update composer packages and untrack composer.lock

* Update code and config for v1.0.0-rc1

* Use contract instead of implementation

* Update README.md

src/Exceptions/ConnectionNotAvailableException.php

@@ -9,7 +9,7 @@ namespace PhpMqtt\Client\Exceptions;
  *
  * @package PhpMqtt\Client\Exceptions
  */
-class ConnectionNotAvailableException extends MQTTClientException
+class ConnectionNotAvailableException extends MqttClientException
 {
     public function __construct(string $name)
     {