Upgrade bucklets

To synchronize the buck version with Gerrit master and
get a fix for the missing test libraries in the classpath.

Change-Id: I952943449c20ea64b7139d85c4fa5250a1de95b7

bucklets

diff --git a/bucklets b/bucklets
index 9c9a68c..6f64640 160000
--- a/bucklets
+++ b/bucklets
@@ -1 +1 @@
-Subproject commit 9c9a68c56b7ce2baa1fcb61774dab887a6b16f12
+Subproject commit 6f64640d35666ecc11fb1cf353f1988407499ce9