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

Change-Id: Ib803fe2cb9131cbe39b3ccbcd75c69573a508371
diff --git a/WORKSPACE b/WORKSPACE
index 598ff2b..d51d148 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -3,7 +3,7 @@
 load("//:bazlets.bzl", "load_bazlets")
 
 load_bazlets(
-    commit = "7ce9a3f58f030635b3a1229b7fb4c62ee7979bc8",
+    commit = "6d3fd710ea4ca44805bb8c858d72b53bd3a500f3",
     # local_path = "/home/<user>/projects/bazlets",
 )