Only mentionify the page if it's an HTML page

Fixes #9. Replaces #10.

Jekyll doesn't provide an `Site#html_pages`. Liquid [`site.html_pages` is constructed here.](https://github.com/jekyll/jekyll/blob/master/lib/jekyll/site.rb#L324)

- [x] Limit mentionifying to just pages whose output extension is `.html`
- [x] Use fixtures for the tests
- [x] Bring Travis fully up to speed with the `Time.now` (present)