Updating CONTRIBUTERS.txt for https://github.com/celery/celery/pull/2790

and https://github.com/celery/celery/pull/2645.

CONTRIBUTORS.txt

@@ -211,3 +211,4 @@ Komu Wairagu, 2016/04/03
 Joe Sanford, 2016/04/11
 Takeshi Kanemoto, 2016/04/22
 Arthur Vuillard, 2016/04/22
+Colin McIntosh, 2016/04/26