Add diff URLs to CHANGELOG
### Added
- Initial release
-[Unreleased]: https://github.com/travis-ci/worker/compare/v2.4.0...HEAD
+[Unreleased]: https://github.com/travis-ci/worker/compare/v2.6.0...HEAD
+[2.6.0]: https://github.com/travis-ci/worker/compare/v2.5.0...v2.6.0
+[2.5.0]: https://github.com/travis-ci/worker/compare/v2.4.0...v2.5.0
[2.4.0]: https://github.com/travis-ci/worker/compare/v2.3.1...v2.4.0
[2.3.1]: https://github.com/travis-ci/worker/compare/v2.3.0...v2.3.1
[2.3.0]: https://github.com/travis-ci/worker/compare/v2.2.0...v2.3.0
