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

Change-Id: I4bc81c597c8832c153b90775d6234d5bdf74dbbc
diff --git a/WORKSPACE b/WORKSPACE
index 3bc6c9d..aae702b 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -3,7 +3,7 @@
 load("//:bazlets.bzl", "load_bazlets")
 
 load_bazlets(
-    commit = "087da1f6ca119ad5c4bc443926426ddf44dcde3a",
+    commit = "b54eaed487d37188120da6933b97c571519954ca",
 )
 
 # Release Plugin API