Updated tk-desktop2 to v1.6.0

Release notes: https://github.com/shotgunsoftware/tk-desktop2/wiki/Release-Notes#v160

Author: toolkitty

env/includes/common/engines.yml

@@ -59,7 +59,7 @@ common.engines.tk-desktop.location:
 common.engines.tk-desktop2.location:
   type: app_store
   name: tk-desktop2
-  version: v1.5.2
+  version: v1.6.0
 common.engines.tk-shell.location:
   type: app_store
   name: tk-shell