[Mailer] fixed undefined index when sending mail

When a Mandrill API request is succesful, it returns an array of
results, one for each recipient. To get rid of the undefined index
error, we grab the message ID from the first recipient in the array.

src/Symfony/Component/Mailer/Bridge/Mailchimp/Transport/MandrillApiTransport.php

@@ -57,7 +57,8 @@ class MandrillApiTransport extends AbstractApiTransport
             throw new HttpTransportException(sprintf('Unable to send an email (code %s).', $result['code']), $response);
         }
 
-        $sentMessage->setMessageId($result['_id']);
+        $firstRecipient = reset($result);
+        $sentMessage->setMessageId($firstRecipient['_id']);
 
         return $response;
     }