Feature: Added support for Send API v3.1

We now support Send API v3.1 !
If you want to use Send API v3.1 you only need to select the proper version => v3.1 .

$clientOptions = ['url' => "api.mailjet.com", 'version' => 'v3.1', 'call' => true];
$transport = new MailjetTransport($dispatchEvent, $apiKey, $apiSecret, $clientOptions);