Fix Symfony 2.3-incompatible calls

Author: ddeboer

src/ProxyClient/AbstractProxyClient.php

@@ -93,7 +93,7 @@ public function flush()
     protected function getDefaultOptions()
     {
         $resolver = new OptionsResolver();
-        $resolver->setDefault('base_uri', null);
+        $resolver->setDefaults(['base_uri' => null]);
 
         return $resolver;
     }

src/ProxyClient/Varnish.php

@@ -196,7 +196,7 @@ public function refresh($url, array $headers = [])
     protected function getDefaultOptions()
     {
         $resolver = parent::getDefaultOptions();
-        $resolver->setDefault('tags_header', 'X-Cache-Tags');
+        $resolver->setDefaults(['tags_header' => 'X-Cache-Tags']);
 
         return $resolver;
     }