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

Change-Id: I68d57cf82c48da46a064d0f6bcbeff8c91b12fee

WORKSPACE

diff --git a/WORKSPACE b/WORKSPACE
index 9b83b2e..6e77106 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",
 )