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

Change-Id: Ib6bcfee2ad04f75b5eb02fc3e946462c0dc7334b

WORKSPACE

diff --git a/WORKSPACE b/WORKSPACE
index 7442807..75d0368 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -3,7 +3,7 @@
 load("//:bazlets.bzl", "load_bazlets")
 
 load_bazlets(
-    commit = "6d3fd710ea4ca44805bb8c858d72b53bd3a500f3",
+    commit = "d4e586bd341207f0b8aafcbe7dbcd4843f852826",
     #local_path = "/home/<user>/projects/bazlets",
 )