Updated tk-desktop to v2.7.9

Release notes: https://github.com/shotgunsoftware/tk-desktop/wiki/Release-Notes#v279

Author: toolkitty

env/includes/common/engines.yml

@@ -50,7 +50,7 @@ common.engines.tk-vred.location:
 common.engines.tk-desktop.location:
   type: app_store
   name: tk-desktop
-  version: v2.7.8
+  version: v2.7.9
 common.engines.tk-desktop2.location:
   type: app_store
   name: tk-desktop2