Update CONTRIBUTORS.txt

As requested in #1792

CONTRIBUTORS.txt

@@ -153,3 +153,4 @@ Vsevolod Kulaga, 2013/11/16
 Ionel Cristian Mărieș, 2013/12/09
 Константин Подшумок, 2013/12/16
 Antoine Legrand, 2014/09/01
+Pepijn de Vos, 2014/15/01