(2.6) setOptional is deprecated in favor of setDefined

Sf <2.6 compatibility is taken care of

Form/Extension/ChoiceTypeExtension.php

@@ -30,7 +30,14 @@ class ChoiceTypeExtension extends AbstractTypeExtension
      */
     public function setDefaultOptions(OptionsResolverInterface $resolver)
     {
-        $resolver->setOptional(array('sortable'));
+        $optionalOptions = array('sortable');
+
+        if (method_exists($resolver, 'setDefined')) {
+            $resolver->setDefined($optionalOptions);
+        } else {
+            // To keep Symfony <2.6 support
+            $resolver->setOptional($optionalOptions);
+        }
     }
 
     /**
@@ -50,4 +57,4 @@ class ChoiceTypeExtension extends AbstractTypeExtension
     {
         return 'choice';
     }
-}
+}