Fixed broken CI

Author: Nyholm

tests/Api/MailboxesTest.php

@@ -11,7 +11,7 @@
 
 namespace Mailgun\Tests\Api;
 
-use GuzzleHttp\Psr7\Response;
+use Nyholm\Psr7\Response;
 use Mailgun\Api\Mailboxes;
 use Mailgun\Exception\InvalidArgumentException;