Upgrade bazlets to latest stable-2.15 to build with 2.15.8 API

Change-Id: I33c38ed29ffbd20469befbf686c24011db5bd9a2
diff --git a/WORKSPACE b/WORKSPACE
index 43094cb..cae3023 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -3,7 +3,7 @@
 load("//:bazlets.bzl", "load_bazlets")
 
 load_bazlets(
-    commit = "b54eaed487d37188120da6933b97c571519954ca",
+    commit = "d4e586bd341207f0b8aafcbe7dbcd4843f852826",
     #local_path = "/home/<user>/projects/bazlets",
 )