Upgrade bazlets to latest stable-3.1 to build with 3.1.8 API

Change-Id: I809f322959e9088f05856a0d919f4cb08cc90f9e

WORKSPACE

diff --git a/WORKSPACE b/WORKSPACE
index 03d2fa5..2218eff 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -3,7 +3,7 @@
 load("//:bazlets.bzl", "load_bazlets")
 
 load_bazlets(
-    commit = "b6cb89690a5e06261932eb30c95fd8db34a7943a",
+    commit = "3f9dadc615dc4053369a42d9ada37dafd8d4763c",
     #local_path = "/home/<user>/projects/bazlets",
 )