Fix user relation while maintaining older verions

There is another approach which is using `app()->version()` however I didn't want to go throw strings manipulation to figure it out!

src/Rateable/Rating.php

@@ -27,6 +27,11 @@ public function rateable()
      */
     public function user()
     {
-        return $this->belongsTo(Config::get('auth.providers.users.model'));
+        $userClassName = Config::get('auth.model');
+        if (is_null($userClassName)) {
+            $userClassName = Config::get('auth.providers.users.model');
+        }
+
+        return $this->belongsTo($userClassName);
     }
 }