Automatically update “last updated” date

I noticed that the “last updated” date was fixed to 28 August, 2012 and hasn’t been updated since. I replaced the date with Jekyll’s `site.time` variable so that it is automatically updated when building the site.

Just for clarification: I used `site.time` instead of `'now'` as `'now'` [doesn’t work with Ruby 1.9.3 and Liquid 2.5.1](https://github.com/Shopify/liquid/pull/117). It has been fixed with Liquid 2.5.2 but `github-pages` still uses 2.5.1 so I went with `site.time`.