Update CONTRIBUTORS.txt

adding myself, https://github.com/celery/celery/issues/2992

CONTRIBUTORS.txt

@@ -200,3 +200,4 @@ Sukrit Khera, 2015/10/26
 Dave Smith, 2015/10/27
 Dennis Brakhane, 2015/10/30
 Chris Harris, 2015/11/27
+Valentyn Klindukh, 2016/01/15