Merge branch 'hotfix/4246'

Close #4246
[2.1.5] Prepare for release

- Updated README to remove dev status, provide date
- Updated `Zend\Version` to remove dev status
- Added 2.1.5 entries to CHANGELOG
Merge branch 'releases/2.1.5'

2.1.5 readiness
Merge branch 'releases/2.1.5' into develop

2.1.5 readiness

Conflicts:
	README.md
	library/Zend/Version/Version.php
Bump to next development version
Merge branch 'version/bump' into develop

Bump to next development version

Conflicts:
	README.md
	library/Zend/Version/Version.php