prepare 6.0.1 release (#589)

bump to version 6.0

prepare 5.3 release (#545)

* update changelog for 5.3 release

* set version to 5.3

* rdoc formatting

* add ruby 2.7 to changelog

Prepare version 5.2.0 (#478)

* Prepare version 5.2.0

* Upgrade database_cleaner for ruby 2.3 compatibility.

Version: 5.1.0

Version 5.0.0

Version v4.1.1

Version v4.1.0

Version 4.0.0

Version 3.5.2