Update changelog for version 7.4.0

Author: Sephster

CHANGELOG.md

@@ -5,10 +5,14 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
 and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
+
+## [7.4.0] - released 2019-05-05
+
 ### Changed
 - RefreshTokenRepository can now return null, allowing refresh tokens to be optional. (PR #649)
 
 ## [7.3.3] - released 2019-03-29
+
 ### Added
 - Added `error_description` to the error payload to improve standards compliance. The contents of this are copied from the existing `message` value. (PR #1006)
 
@@ -441,7 +445,8 @@ Version 5 is a complete code rewrite.
 
 - First major release
 
-[Unreleased]: https://github.com/thephpleague/oauth2-server/compare/7.3.3...HEAD
+[Unreleased]: https://github.com/thephpleague/oauth2-server/compare/7.4.0...HEAD
+[7.4.0]: https://github.com/thephpleague/oauth2-server/compare/7.3.3...7.4.0
 [7.3.3]: https://github.com/thephpleague/oauth2-server/compare/7.3.2...7.3.3
 [7.3.2]: https://github.com/thephpleague/oauth2-server/compare/7.3.1...7.3.2
 [7.3.1]: https://github.com/thephpleague/oauth2-server/compare/7.3.0...7.3.1