New entries for 7.4.0

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

CHANGELOG

+v 7.4.0
+  - Refactored membership logic
+  - Fix creating new files with web editor
+  - Improve error reporting on users API (Julien Bianchi)
+  - Refactor test coverage tools usage. Use SIMPLECOV=true to generate it locally
+  - Increase unicorn timeout to 60 seconds
+
 v 7.3.1
   - Fix ref parsing in Gitlab::GitAccess
   - Fix error 500 when viewing diff on a file with changed permissions