Add missing property 'avatar_url' to Project model

Author: benito103e

lib/Gitlab/Model/Project.php

@@ -29,6 +29,7 @@
  * @property-read int $greatest_access_level
  * @property-read string $last_activity_at
  * @property-read string $tag_list
+ * @property-read string $avatar_url
  * @property-read User $owner
  * @property-read ProjectNamespace $namespace
  */
@@ -61,7 +62,8 @@ class Project extends AbstractModel
         'greatest_access_level',
         'last_activity_at',
         'snippets_enabled',
-        'tag_list'
+        'tag_list',
+        'avatar_url'
     );
 
     /**