Upgrade bazlets to latest stable-2.15

download_file.py: Fix root workspace directory detection

Change-Id: I1c5eb23dfe5978270c10ef6d2baaed7ecd0a04fc

WORKSPACE

diff --git a/WORKSPACE b/WORKSPACE
index ee955f3..96bada7 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -3,7 +3,7 @@
 load("//:bazlets.bzl", "load_bazlets")
 
 load_bazlets(
-    commit = "9066deaedaf97c619c3de31fefae542ccfc62b7d",
+    commit = "d735ef54dfd38f99106d5635228156139c0b57fd",
     #local_path = "/home/<user>/projects/bazlets",
 )