Upgrade bazlets to latest stable-2.15 to build with 2.15.15 API

Change-Id: Ib6713e3d1ff40df6b9751d431017b28f0de2f020
diff --git a/WORKSPACE b/WORKSPACE
index 1b5b63d..b263940 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -3,7 +3,7 @@
 load("//:bazlets.bzl", "load_bazlets")
 
 load_bazlets(
-    commit = "d60efafe2fa2581d8b6f10aeb80beef5a77b8093",
+    commit = "d710dc4d8441475fbedae2171cf3cbafe6d80f23",
     #local_path = "/home/<user>/projects/bazlets",
 )