update barracuda to 0.7.1 (#3977)

com.unity.ml-agents/CHANGELOG.md

 #### com.unity.ml-agents (C#)
 - A bug that would cause the editor to go into a loop when a prefab was selected was fixed. (#3949)
 - BrainParameters.ToProto() no longer throws an exception if none of the fields have been set. (#3930)
+- The Barracuda dependency was upgraded to 0.7.0-preview. (#3977)
 #### ml-agents / ml-agents-envs / gym-unity (Python)
 - An issue was fixed where using `--initialize-from` would resume from the past step count. (#3962)
 - The gym wrapper error for the wrong number of agents now fires more consistently, and more details

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": "0.7.0-preview"
+    "com.unity.barracuda": "0.7.1-preview"
-}
+}