Upgrade bazlets to latest stable-3.1 to build with 3.1.5 API

Change-Id: Ib608498e2883aff6ceea8a0b36b17eaeb48ec462

WORKSPACE

diff --git a/WORKSPACE b/WORKSPACE
index dcb4416..7cf9808 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -3,7 +3,7 @@
 load("//:bazlets.bzl", "load_bazlets")
 
 load_bazlets(
-    commit = "fc1bfdf008c4b07e7cc4214fdacf25560276f9c4",
+    commit = "c6656e6a3ed76eb7e6d9a217cbe5d59c415efb9a",
     #local_path = "/home/<user>/projects/bazlets",
 )