Fix link and id of the feed

Currently, the hosts of link and id of [the feed](http://rubini.us/feed/atom.xml) are `0.0.0.0`.

This PR replaces `{{ site.host }}`, which returns `0.0.0.0`, with `{{ site.url }}`, which returns `http://rubini.us` as defined in [_config.yml](https://github.com/rubinius/rubinius.github.io/blob/master/_config.yml).