Fix usage of deprecated method Yaml::load

lib/Gedmo/Sortable/Mapping/Driver/Yaml.php

@@ -81,7 +81,7 @@ class Yaml extends File implements Driver
      */
     protected function _loadMappingFile($file)
     {
-        return \Symfony\Component\Yaml\Yaml::load($file);
+        return \Symfony\Component\Yaml\Yaml::parse($file);
     }
 
     /**