Added Alan Justino da Silva to CONTRIBUTORS.txt (#3739)

As asked at https://github.com/celery/celery/pull/3480#issuecomment-253951353, added myself. Used the date of the request. Please modify if needed.

CONTRIBUTORS.txt

@@ -219,6 +219,7 @@ Kevin Richardson, 2016/06/29
 Andrew Stewart, 2016/07/04
 Xin Li, 2016/08/03
 Alli Witheford, 2016/09/29
+Alan Justino da Silva, 2016/10/14
 Marat Sharafutdinov, 2016/11/04
 Viktor Holmqvist, 2016/12/02
 Rick Wargo, 2016/12/02