[barracuda] Bump version to 0.6.3. (#3766)

com.unity.ml-agents/CHANGELOG.md

  - Fixed an issue where exceptions from environments provided a returncode of 0. (#3680)
  - Running `mlagents-learn` with the same `--run-id` twice will no longer overwrite the existing files. (#3705)
  - `StackingSensor` was changed from `internal` visibility to `public`
+ - Updated Barracuda to 0.6.3-preview.
 
 ## [0.15.1-preview] - 2020-03-30
 ### Bug Fixes

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": "0.6.1-preview"
+    "com.unity.barracuda": "0.6.3-preview"
   }
 }