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

Change-Id: I7f419582139cffcb43f8476946b44d4c629cf4de

WORKSPACE

diff --git a/WORKSPACE b/WORKSPACE
index 1c2385f..e0ab930 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",
 )