Use h2 for version headings to allow deeplinking

Amended the changelog to use Markdown-style h2 headings for the versions. This allows [deeplinking](https://github.com/colszowka/ansible/blob/patch-1/CHANGELOG.md#134-top-of-the-world-reprise---october-29-2013) release notes for specific versions thanks to [html-pipeline](https://github.com/jch/html-pipeline). Yay :)