Fix access level of addColumn

src/Jenssegers/Mongodb/Schema/Blueprint.php

@@ -207,7 +207,7 @@ class Blueprint extends \Illuminate\Database\Schema\Blueprint
      * @param  array   $parameters
      * @return Blueprint
      */
-    protected function addColumn($type, $name, array $parameters = [])
+    public function addColumn($type, $name, array $parameters = [])
     {
         $this->fluent($name);