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

Change-Id: I25a23718ed9e2173087ac9fe58d64206bff8a1eb

WORKSPACE

diff --git a/WORKSPACE b/WORKSPACE
index 7009bfd..c9a692b 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -3,7 +3,7 @@
 load("//:bazlets.bzl", "load_bazlets")
 
 load_bazlets(
-    commit = "f1d3eefb78029298afe119b0d8b2a43de2b510f6",
+    commit = "d4e586bd341207f0b8aafcbe7dbcd4843f852826",
     #local_path = "/home/<user>/projects/bazlets",
 )