Upgrade bazlets to latest stable-2.15 to build with 2.15.19 API

Change-Id: I91decb69e0d11afdf61787b3a0f48dce066fd58d

WORKSPACE

diff --git a/WORKSPACE b/WORKSPACE
index d9534d5..3359311 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -3,7 +3,7 @@
 load("//:bazlets.bzl", "load_bazlets")
 
 load_bazlets(
-    commit = "f53f51fb660552d0581aa0ba52c3836ed63d56a3",
+    commit = "9af263722b7eafe99af079d6ef7cf1de23e6f8d7",
     #local_path = "/home/<user>/projects/bazlets",
 )