Upgrade bazlets to latest stable-2.15 to build with 2.15.7 API

Change-Id: I5d55d86589a38ae1a0087ce240b4e4db63c546bc

WORKSPACE

diff --git a/WORKSPACE b/WORKSPACE
index 7cc94a5..cff9d9e 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -3,7 +3,7 @@
 load("//:bazlets.bzl", "load_bazlets")
 
 load_bazlets(
-    commit = "087da1f6ca119ad5c4bc443926426ddf44dcde3a",
+    commit = "b54eaed487d37188120da6933b97c571519954ca",
     #local_path = "/home/<user>/projects/bazlets",
 )