Use snapshots for posts instead of two versions

You're using two versions for posts, and one version has no routing information associated. This is not a problem on it's own, but if you load the version without routing information, `$url$` will not be set.

While it's fixable using two versions, I think snapshots are a better fit here. You can find a tutorial on shapshots here: http://jaspervdj.be/hakyll/tutorials/05-snapshots-feeds.html