Upgrade bazlets to latest stable-2.15 to build with 2.15.16 API

Change-Id: I930c5639f4586d5e17e707c09109e93db0f10c7c

WORKSPACE

diff --git a/WORKSPACE b/WORKSPACE
index 0b5eb65..29e995c 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -3,7 +3,7 @@
 load("//:bazlets.bzl", "load_bazlets")
 
 load_bazlets(
-    commit = "d710dc4d8441475fbedae2171cf3cbafe6d80f23",
+    commit = "4b3ce19d8d18d9f7df56a06532723b91ea6949d0",
     #local_path = "/home/<user>/projects/bazlets",
 )