Fixed issue where loc blocks are generated incorrectly

This is a fix for [issue 334](https://github.com/dart-lang/dartlang.org/issues/334), hence the branch name.

Michael Levins [sitemap_plugin.rb](https://github.com/kinnetica/jekyll-plugins/blob/master/sitemap_generator.rb) (linked from the official [jekyll site](http://jekyllrb.com/docs/plugins/)) suggests that the @dir attribute isn't needed in location_on_server. 

I've tested the sitemap generation locally with success.