Upgrade bazlets to latest stable-2.15 revision

Change-Id: Ie6a32709e48134cadd3bdb0ff5b6f0d655ad93f9
diff --git a/WORKSPACE b/WORKSPACE
index 109865d..6cae858 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -3,7 +3,7 @@
 load("//:bazlets.bzl", "load_bazlets")
 
 load_bazlets(
-    commit = "60296e8f7162f279e3885a92014bc69252935612",
+    commit = "4a4cff4b437e60abd38c6229fabd79692e7fbd7d",
     #    local_path = "/home/<user>/projects/bazlets",
 )