Upgrade bazlets to latest stable-2.15 to build with 2.15.10 API

Change-Id: I57f71a6e6017a88f12a32e8ce6fed995f01acbcb
diff --git a/WORKSPACE b/WORKSPACE
index c6153f6..5e7f435 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -3,7 +3,7 @@
 load("//:bazlets.bzl", "load_bazlets")
 
 load_bazlets(
-    commit = "ca34f0cf89b2e041ea7f4aa4b9696efc2d76746f",
+    commit = "2b1d68959119920e5fa9bdfb9f0cf926bfef4929",
     #local_path = "/home/<user>/projects/bazlets",
 )