Update buck version to match Gerrit master branch

Change-Id: I3b3037b57c2bccfda076496bd91934b9349258d1

buckversion

diff --git a/buckversion b/buckversion
index af38772..560aff2 100644
--- a/buckversion
+++ b/buckversion
@@ -1 +1 @@
-7b7817c48f30687781040b2b82ac9218d5c4eaa4
+d6949e1440ef2048d697c637a4adae1b509bf72d