[barracuda] Update barracuda to 0.6.0-preview. (#3444)

com.unity.ml-agents/CHANGELOG.md

  - Several classes were changed from public to internal visibility. (#3390)
  - Academy.RegisterSideChannel and UnregisterSideChannel methods were added. (#3391)
  - A tutorial on adding custom SideChannels was added (#3391)
+ - Update Barracuda to 0.6.0-preview
 
 ### Bugfixes
 

com.unity.ml-agents/package.json

 	"unity": "2018.4",
 	"description": "Add interactivity to your game with Machine Learning Agents trained using Deep Reinforcement Learning.",
 	"dependencies": {
-		"com.unity.barracuda": "0.5.0-preview"
+		"com.unity.barracuda": "0.6.0-preview"
 	}
 }