`values()` function did not return the object, this breaking the fluent interface.

Added a `return $this` so that usage of this node works as expected:
    ->enumNode('foo')->values(array('a', 'b'))->end()

src/Symfony/Component/Config/Definition/Builder/EnumNodeDefinition.php

@@ -23,6 +23,8 @@ class EnumNodeDefinition extends ScalarNodeDefinition
         }
 
         $this->values = $values;
+        
+        return $this;
     }
 
     /**
@@ -38,4 +40,4 @@ class EnumNodeDefinition extends ScalarNodeDefinition
 
         return new EnumNode($this->name, $this->parent, $this->values);
     }
-}
+}