Add restrictOnUpdate() method to the schema builder

Hi,

Laravel has a `cascadeOnUpdate()` but does not have a `restrictOnUpdate()`.

I know that i can use `onUpdate('restrict')` but since laravel already has a helper for the cascade and also has a restrict helper for the delete, it only makes sense to add helper for the restrict on update as well.

src/Illuminate/Database/Schema/ForeignKeyDefinition.php

@@ -34,6 +34,16 @@ public function cascadeOnDelete()
         return $this->onDelete('cascade');
     }
 
+    /**
+     * Indicate that updates should be restricted.
+     *
+     * @return $this
+     */
+    public function restrictOnUpdate()
+    {
+        return $this->onUpdate('restrict');
+    }
+
     /**
      * Indicate that deletes should be restricted.
      *