Upgrade bazlets to latest stable-2.15 to build with 2.15.5 API

Change-Id: I1db38e28097df7baa5ea343d7c9bef9c565c8bfd

WORKSPACE

diff --git a/WORKSPACE b/WORKSPACE
index 3517e8b..2704b12 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -3,7 +3,7 @@
 load("//:bazlets.bzl", "load_bazlets")
 
 load_bazlets(
-    commit = "88570121c377a8a94c76915566114bf02113a7d9",
+    commit = "6d3fd710ea4ca44805bb8c858d72b53bd3a500f3",
     # local_path = "/home/<user>/projects/bazlets",
 )