Updated tk-vred to v3.5.6

Release notes: https://github.com/shotgunsoftware/tk-vred/releases/tag/v3.5.6

Author: toolkitty-app[bot]

env/includes/engine_locations.yml

@@ -72,4 +72,4 @@ engines.tk-shotgun.location:
 engines.tk-vred.location:
   type: app_store
   name: tk-vred
-  version: v3.5.5
+  version: v3.5.6