Added no-cache header

This prevents Varnish from caching the captcha image

Controller/CaptchaController.php

@@ -53,6 +53,7 @@ class CaptchaController extends Controller
 
         $response = new Response($generator->generate($options));
         $response->headers->set('Content-type', 'image/jpeg');
+        $response->headers->set('Pragma', 'no-cache');
 
         return $response;
     }