Upgrade buck to latest version

Removes dependency on `realpath` which is not available by default
on OSX.

This upgrades to the same version as used on Gerrit master.

Change-Id: Ib6ec24d9c097cd63b3e50faee4acd5e1fd638dbe

buckversion

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