Fixed requires with __PATH__

This because the gateway had issues with an autoloader or such in
WHMCS. Don't know the exact cause, but this fixed it.

src/molliebanktransfer.php

@@ -1,6 +1,6 @@
 <?php
 
-require_once 'mollie/mollie.php';
+require_once __DIR__ . '/mollie/mollie.php';
 
 function molliebanktransfer_config() {
 	$config = mollie_config();