update changelog links

Author: barryorourke

CHANGELOG.md

@@ -38,7 +38,8 @@ This CHANGELOG follows the format listed [here](https://github.com/sensu-plugins
 ## [0.1.0] 2014-01-06
 - Initial release (@zsprackett)
 
-[Unreleased]: https://github.com/sensu-plugins/sensu-plugin-python/compare/8920afcda62b34e9134ba9a816582dbf5f52806c...HEAD
+[Unreleased]: https://github.com/sensu-plugins/sensu-plugin-python/compare/0.4.0...HEAD
+[0.4.0]: https://github.com/sensu-plugins/sensu-plugin-python/compare/8920afcda62b34e9134ba9a816582dbf5f52806c...0.4.0
 [0.3.2]: https://github.com/sensu-plugins/sensu-plugin-python/compare/40314082947208acf9ed7c6d6c321ea52a14e765...8920afcda62b34e9134ba9a816582dbf5f52806c
 [0.3.1]: https://github.com/sensu-plugins/sensu-plugin-python/compare/2deaf3a34cd86afe13af9ab34aefd8056d284e85...40314082947208acf9ed7c6d6c321ea52a14e765
 [0.3.0]: https://github.com/sensu-plugins/sensu-plugin-python/compare/1302599c366ce30e04119bbc7551a258b33a7eab...2deaf3a34cd86afe13af9ab34aefd8056d284e85