Update bazlets to latest stable-2.15 and build with released 2.15.4 API

Change-Id: I1a77bdfa0ee192663b4fb6e701ecc52ba58931ba
diff --git a/WORKSPACE b/WORKSPACE
index 6cae858..4a19169 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -3,7 +3,7 @@
 load("//:bazlets.bzl", "load_bazlets")
 
 load_bazlets(
-    commit = "4a4cff4b437e60abd38c6229fabd79692e7fbd7d",
+    commit = "88570121c377a8a94c76915566114bf02113a7d9",
     #    local_path = "/home/<user>/projects/bazlets",
 )