set package and API to 0.15.0-dev0 (#3369)

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

         "docs/Learning-Environment-Design.md")]
     public class Academy : IDisposable
     {
-        const string k_ApiVersion = "API-14";
+        const string k_ApiVersion = "API-15-dev0";
         const int k_EditorTrainingPort = 5004;
 
         // Lazy initializer pattern, see https://csharpindepth.com/articles/singleton#lazy

gym-unity/gym_unity/__init__.py

-__version__ = "0.14.0"
+__version__ = "0.15.0.dev0"

ml-agents-envs/mlagents_envs/__init__.py

-__version__ = "0.14.0"
+__version__ = "0.15.0.dev0"

ml-agents-envs/mlagents_envs/environment.py

 class UnityEnvironment(BaseEnv):
     SCALAR_ACTION_TYPES = (int, np.int32, np.int64, float, np.float32, np.float64)
     SINGLE_BRAIN_ACTION_TYPES = SCALAR_ACTION_TYPES + (list, np.ndarray)
-    API_VERSION = "API-14"
+    API_VERSION = "API-15-dev0"
 
     def __init__(
         self,

ml-agents/mlagents/trainers/__init__.py

-__version__ = "0.14.0"
+__version__ = "0.15.0.dev0"