Fix feed link in feed.xml

Recently we had a wrong link in feed.xml, because the link contained an
output directory path. For example, we had the following XML-node:

    <link href="http://kalnitsky.org/_build/feed.xml" rel="self"/>

when we're waiting for:

 <link href="http://kalnitsky.org/feed.xml" rel="self"/>

Since now it's fixed.

Fixed #60