Upgrade bazlets to latest stable-2.15

To use the now mandatory https protocol for MAVEN_CENTRAL.

Bug: Issue 12170
Change-Id: I6dc1a0e4b53e64134d128f9d8857eaf73d12b258

WORKSPACE

diff --git a/WORKSPACE b/WORKSPACE
index b594ad3..529d53c 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -3,7 +3,7 @@
 load("//:bazlets.bzl", "load_bazlets")
 
 load_bazlets(
-    commit = "ec949feac1b9dae6cc2c8c25d254f34924c54296",
+    commit = "f53f51fb660552d0581aa0ba52c3836ed63d56a3",
     #local_path = "/home/<user>/projects/bazlets",
 )