Version bump main (#5279)

* Version bump for the main branch following branching of release_17

* Messed up the extensions package version

com.unity.ml-agents.extensions/package.json

 {
   "name": "com.unity.ml-agents.extensions",
   "displayName": "ML Agents Extensions",
-  "version": "0.3.1-preview",
+  "version": "0.4.0-preview",
   "unity": "2019.4",
   "description": "A source-only package for new features based on ML-Agents",
   "dependencies": {

com.unity.ml-agents/CHANGELOG.md

 and this project adheres to
 [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+### Major Changes
+### Minor Changes
+### Bug Fixes
+
 
 ## [2.0.0-exp.1] - 2021-04-22
 ### Major Changes

gym-unity/gym_unity/__init__.py

 # Version of the library that will be used to upload to pypi
-__version__ = "0.26.0.dev0"
+__version__ = "0.27.0.dev0"
 
 # Git tag that will be checked to determine whether to trigger upload to pypi
 __release_tag__ = None

ml-agents-envs/mlagents_envs/__init__.py

 # Version of the library that will be used to upload to pypi
-__version__ = "0.26.0.dev0"
+__version__ = "0.27.0.dev0"
 
 # Git tag that will be checked to determine whether to trigger upload to pypi
 __release_tag__ = None

ml-agents/mlagents/trainers/__init__.py

 # Version of the library that will be used to upload to pypi
-__version__ = "0.26.0.dev0"
+__version__ = "0.27.0.dev0"
 
 # Git tag that will be checked to determine whether to trigger upload to pypi
 __release_tag__ = None