Upgrade bazlets to latest stable-2.15 to build with 2.15.12 API

Change-Id: I320a4d2cebdd1d492950364294bfde8c5ffe638a

WORKSPACE

diff --git a/WORKSPACE b/WORKSPACE
index 60f93ed..2f74d5e 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -3,7 +3,7 @@
 load("//:bazlets.bzl", "load_bazlets")
 
 load_bazlets(
-    commit = "9c84d2870bf8d3f51685a289a1fc9edc3bf9dc89",
+    commit = "f4fcc606a6afa8ce27a013bcf62e495a5ec2505c",
     #local_path = "/home/<user>/projects/bazlets",
 )