Update barracuda, swtich Agents in Sorter use Burst. (#4979)

Project/Assets/ML-Agents/Examples/Sorter/Prefabs/Area.prefab

     VectorActionSpaceType: 0
     hasUpgradedBrainParametersWithActionSpec: 1
   m_Model: {fileID: 11400000, guid: a2b17bcb0df4d448893e800c34d87c4c, type: 3}
-  m_InferenceDevice: 0
+  m_InferenceDevice: 2
   m_BehaviorType: 0
   m_BehaviorName: Sorter
   TeamId: 0

com.unity.ml-agents/package.json

   "unity": "2018.4",
   "description": "Use state-of-the-art machine learning to create intelligent character behaviors in any Unity environment (games, robotics, film, etc.).",
   "dependencies": {
-    "com.unity.barracuda": "1.3.0-preview",
+    "com.unity.barracuda": "1.3.1-preview",
     "com.unity.modules.imageconversion": "1.0.0",
     "com.unity.modules.jsonserialize": "1.0.0",
     "com.unity.modules.physics": "1.0.0",