Bumping version on the release (#3615)

com.unity.ml-agents/Runtime/Academy.cs

         /// Unity package version of com.unity.ml-agents.
         /// This must match the version string in package.json and is checked in a unit test.
         /// </summary>
-        internal const string k_PackageVersion = "0.14.1-preview";
+        internal const string k_PackageVersion = "0.15.0-preview";
 
         const int k_EditorTrainingPort = 5004;
 

com.unity.ml-agents/package.json

 {
-	"name": "com.unity.ml-agents",
-	"displayName":"ML Agents",
-	"version": "0.14.1-preview",
-	"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"
-	}
-}
+  "name": "com.unity.ml-agents",
+  "displayName": "ML Agents",
+  "version": "0.15.0-preview",
+  "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"
+  }
+}

gym-unity/gym_unity/__init__.py

-__version__ = "0.15.0.dev0"
+__version__ = "0.15.0"

ml-agents-envs/mlagents_envs/__init__.py

-__version__ = "0.15.0.dev0"
+__version__ = "0.15.0"

ml-agents/mlagents/trainers/__init__.py

-__version__ = "0.15.0.dev0"
+__version__ = "0.15.0"