Fix the last-node problem.

I got some help from [1], which prompted me to learn more, and finally
[2] helped me to fix it using just the CSS pseudo-class selector.

[1] http://stackoverflow.com/questions/14922247/how-to-get-a-tree-in-html-using-pure-css
[2] http://net.tutsplus.com/tutorials/html-css-techniques/the-30-css-selectors-you-must-memorize/