Updated tk-motionbuilder to v0.8.8

Release notes: https://github.com/shotgunsoftware/tk-motionbuilder/releases/tag/v0.8.8

Author: toolkitty-app[bot]

env/includes/engine_locations.yml

@@ -52,7 +52,7 @@ engines.tk-maya.location:
 engines.tk-motionbuilder.location:
   type: app_store
   name: tk-motionbuilder
-  version: v0.8.7
+  version: v0.8.8
 engines.tk-nuke.location:
   type: app_store
   name: tk-nuke