Upgrade bazlets to latest stable-3.1 to build with 3.1.7 API

Change-Id: Ie698aed37516dd1e922e7592527929fe0860bef8

WORKSPACE

diff --git a/WORKSPACE b/WORKSPACE
index 599aad6..150486d 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -3,7 +3,7 @@
 load("//:bazlets.bzl", "load_bazlets")
 
 load_bazlets(
-    commit = "bd8db0cf3057397bcf7287c28cc93886e663989b",
+    commit = "b6cb89690a5e06261932eb30c95fd8db34a7943a",
     #local_path = "/home/<user>/projects/bazlets",
 )