Added missing property 'start_date' to Milestone model

Author: Stanislav Gamayunov

lib/Gitlab/Model/Milestone.php

@@ -11,6 +11,7 @@
  * @property-read string $title
  * @property-read string $description
  * @property-read string $due_date
+ * @property-read string $start_date
  * @property-read string $state
  * @property-read bool $closed
  * @property-read string $updated_at
@@ -30,6 +31,7 @@ class Milestone extends AbstractModel
         'title',
         'description',
         'due_date',
+        'start_date',
         'state',
         'closed',
         'updated_at',