Update buck to latest version

In the latest version the use of realpath is removed since it's not
available by default on macOS.

Change-Id: I4dbe401adbb3beba1042579efeabc60c343c720c

.buckversion

diff --git a/.buckversion b/.buckversion
index 560aff2..7eb591f 100644
--- a/.buckversion
+++ b/.buckversion
@@ -1 +1 @@
-d6949e1440ef2048d697c637a4adae1b509bf72d
+e27df656657f93f8d57a7aaac69a5ae0e298a292