Update buck to latest version

This includes a fix for test dependencies not being included
in the output of 'buck audit classpath' [1].

[1] https://github.com/facebook/buck/issues/888

Change-Id: I801454b4d88b196955ad8b33dfb116a43c6e3bea
diff --git a/buckversion b/buckversion
index efb68ec..af38772 100644
--- a/buckversion
+++ b/buckversion
@@ -1 +1 @@
-fd3105a0b62899f74662f4cdc156de6990bdc24c
+7b7817c48f30687781040b2b82ac9218d5c4eaa4