[2.1.1] Prepare for 2.1.1 release

- Updated README.md with date
- Updated Zend\Version\Version::VERSION
- Updated CHANGELOG.md
Merge branch 'releases/2.1.1'

Ready for 2.1.1 release
Bump version to 2.1.2dev
Merge branch 'version/bump'

Bump to 2.1.2dev