Comply with (Gemnasium) conventions for changelogs

Rename History.md to CHANGELOG.md and use first level headers for version numbers.

https://github.com/tech-angels/vandamme#changelogs-convention

I'm not going to use your otherwise promising gem unless the changelog is shown in its [Gemnasium page](https://gemnasium.com/gems/jekyll-minibundle)! ;-)