Update link to release-1.4.15-stable.tar.gz

Firstly by some reason there is no release-1.4.15-stable tag, though I have it
locally, so I pushed it to libevent/libevent@release-1.4.15-stable. After this
we need to update links since releases are in "archive" not in "downloads"

P.S. we also need to fix *.sig (by attaching it to release?)

Refs: libevent/libevent#312

Author: azat

index.html

@@ -158,7 +158,7 @@ <h3>Download&ndash;Stable releases</h3>
        <li>and more...
       </ul>
 
-	<li><a href="https://github.com/downloads/libevent/libevent/libevent-1.4.15-stable.tar.gz">libevent-1.4.15-stable.tar.gz</a> [<a href="https://github.com/downloads/libevent/libevent/libevent-1.4.15-stable.tar.gz.sig">GPG Sig</a>] <a href="https://raw.github.com/libevent/libevent/release-1.4.15-stable/ChangeLog">ChangeLog</a><br>Released 2015-01-05
+	<li><a href="https://github.com/libevent/libevent/archive/release-1.4.15-stable.tar.gz">libevent-1.4.15-stable.tar.gz</a> [<a href="https://github.com/downloads/libevent/libevent/libevent-1.4.15-stable.tar.gz.sig">GPG Sig</a>] <a href="https://raw.github.com/libevent/libevent/release-1.4.15-stable/ChangeLog">ChangeLog</a><br>Released 2015-01-05
           <ul>
             <li>Changelog to follow
            </ul>