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

Change-Id: Ia1fb1a5a454deabaf893ba64b52025463caa9f36

WORKSPACE

diff --git a/WORKSPACE b/WORKSPACE
index 026e89a..e6fda53 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",
 )