Fix RSS links in blog.xml

I noticed the blog RSS feed is broken. The links point to `http://blog.travis-ci.com//2014-02-12-send-your-build-notifications-to-sqwiggle` and similar (note the double slash), so I went ahead and fixed it.

The reason is that Jekyll's post#url is always prefixed with a slash, so including one before it breaks the URL.

See the relevant code:
* [Post#url](https://github.com/jekyll/jekyll/blob/master/lib/jekyll/post.rb#L196)
* [URL#sanitize_url](https://github.com/jekyll/jekyll/blob/master/lib/jekyll/url.rb#L64)

and [the documentation](http://jekyllrb.com/docs/variables/#page_variables).