Upgrade bazlets to latest stable-2.15

- Avoid using deprecated depset union

Change-Id: Ie3a94fa1bae439431f7f1f0cae13806500d48a58
diff --git a/WORKSPACE b/WORKSPACE
index d360e0c..7c73991 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -3,7 +3,7 @@
 load("//:bazlets.bzl", "load_bazlets")
 
 load_bazlets(
-    commit = "52c0a07ea4c41dca2bafbcc142d42ed33785628f",
+    commit = "9c84d2870bf8d3f51685a289a1fc9edc3bf9dc89",
     #local_path = "/home/<user>/projects/bazlets",
 )