Update bazlets to stable-2.15 to build with 2.15.4 API

Change-Id: I6bff3a05f5579160ee72d21a000336a624c113f0
diff --git a/WORKSPACE b/WORKSPACE
index fcaf719..c18df04 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -3,7 +3,7 @@
 load("//:bazlets.bzl", "load_bazlets")
 
 load_bazlets(
-    commit = "8faa61d19d53cc125ac7fd51eb213a496ab14f8d",
+    commit = "88570121c377a8a94c76915566114bf02113a7d9",
     #    local_path = "/home/<user>/projects/bazlets",
 )