Update Slimdown.php

[samuelhaeussler]

Remove self-closing tags in void elements, to be consistent with e. g.

[lux]

Looks like this causes these:

-----

To turn into this:

-<hr />
<ul>
	<li>----</li>
</ul>

I wonder if moving the ul and li parsing below the horizontal rule line might fix that and get tests to pass.