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

Change-Id: I670dd910bee641820ee9bb49bcd4899b49f991c1
diff --git a/WORKSPACE b/WORKSPACE
index b57d59c..99fd47d 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -3,7 +3,7 @@
 load("//:bazlets.bzl", "load_bazlets")
 
 load_bazlets(
-    commit = "3995849fe742201d93e40f2b109dcff13dd0a4ed",
+    commit = "88570121c377a8a94c76915566114bf02113a7d9",
     #    local_path = "/home/<user>/projects/bazlets",
 )