Update structs plugin version.

Version >= 1.23 is required for the bundled publish-over-ssh plugin.

The latest version is 308.v852b473a2b8c which is of odd enough
construction that it looks like it could be a misconfigured auto
release so I'm avoiding it for now.

attributes/plugins.rb

@@ -92,7 +92,7 @@
   "ssh-agent" => "1.17",
   "ssh-credentials" => "1.18.1",
   "ssh-slaves" => "1.28.1",
-  "structs" => "1.22",
+  "structs" => "1.24",
   "subversion" => "2.14.0",
   "swarm" => "3.22",
   "systemloadaverage-monitor" => "1.2",