[2.3.0] Updated VERSION, README, CHANGELOG

- Updated `Zend\Version\Version::VERSION` to 2.3.0 (stable)
- Updated README with release date, non-dev label
- Added 2.3.0 changes to CHANGELOG
Merge branch 'releases/2.3.0'

2.3.0 readiness
Bump version to 2.3.1dev

- Updated README.md
- Bumped `Zend\Version\Version::VERSION`
Merge branch 'version/bump'

Bump to 2.3.1dev