Fix links in changelog.

Author: bheisler

CHANGELOG.md

@@ -230,7 +230,7 @@ more details
 - Initial release on Crates.io.
 
 
-[Unreleased]: https://github.com/bheisler/criterion.rs/compare/0.2.7...HEAD
+[Unreleased]: https://github.com/bheisler/criterion.rs/compare/0.2.8...HEAD
 [0.1.1]: https://github.com/bheisler/criterion.rs/compare/0.1.0...0.1.1
 [0.1.2]: https://github.com/bheisler/criterion.rs/compare/0.1.1...0.1.2
 [0.2.0]: https://github.com/bheisler/criterion.rs/compare/0.1.2...0.2.0
@@ -240,4 +240,5 @@ more details
 [0.2.4]: https://github.com/bheisler/criterion.rs/compare/0.2.3...0.2.4
 [0.2.5]: https://github.com/bheisler/criterion.rs/compare/0.2.4...0.2.5
 [0.2.6]: https://github.com/bheisler/criterion.rs/compare/0.2.5...0.2.6
-[0.2.7]: https://github.com/bheisler/criterion.rs/compare/0.2.5...0.2.7
+[0.2.7]: https://github.com/bheisler/criterion.rs/compare/0.2.6...0.2.7
+[0.2.8]: https://github.com/bheisler/criterion.rs/compare/0.2.7...0.2.8