Merge branch 'master' of github.com:celery/celery

CONTRIBUTORS.txt

@@ -209,3 +209,5 @@ Maxime Verger, 2016/02/29
 Alexander Oblovatniy, 2016/03/10
 Komu Wairagu, 2016/04/03
 Joe Sanford, 2016/04/11
+Takeshi Kanemoto, 2016/04/22
+Arthur Vuillard, 2016/04/22