fix release versions and changelog section

com.unity.ml-agents/CHANGELOG.md

 and this project adheres to
 [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+### Major Changes
+#### com.unity.ml-agents / com.unity.ml-agents.extensions (C#)
+#### ml-agents / ml-agents-envs / gym-unity (Python)
+### Minor Changes
+#### com.unity.ml-agents / com.unity.ml-agents.extensions (C#)
+#### ml-agents / ml-agents-envs / gym-unity (Python)
+### Bug Fixes
+#### com.unity.ml-agents / com.unity.ml-agents.extensions (C#)
+#### ml-agents / ml-agents-envs / gym-unity (Python)
 
 ## [2.1.0-exp.1] - 2021-06-09
 ### Minor Changes

gym-unity/gym_unity/__init__.py

 # Version of the library that will be used to upload to pypi
-__version__ = "0.27.0"
+__version__ = "0.28.0.dev0"
-__release_tag__ = "release_18"
+__release_tag__ = None

ml-agents-envs/mlagents_envs/__init__.py

 # Version of the library that will be used to upload to pypi
-__version__ = "0.27.0"
+__version__ = "0.28.0.dev0"
-__release_tag__ = "release_18"
+__release_tag__ = None

ml-agents/mlagents/trainers/__init__.py

 # Version of the library that will be used to upload to pypi
-__version__ = "0.27.0"
+__version__ = "0.28.0.dev0"
-__release_tag__ = "release_18"
+__release_tag__ = None