Add Debian logo to downloads page

Source:
https://www.debian.org/logos/openlogo-nd-100.png

source/downloads/index.html

@@ -32,7 +32,7 @@ title: GitLab Community Edition (CE) Downloads
                     <td>93eb68aa407a33f01b376308bde0b465</td>
                   </tr>
                   <tr>
-                    <td><img src="/images/ubuntu.png"></td>
+                    <td><img src="/images/debian.png"></td>
                     <td>Debian 7 (Wheezy) 64bit</td>
                     <td><a href="https://downloads-packages.s3.amazonaws.com/debian-7.4/gitlab_6.8.1-omnibus.4-1_amd64.deb">gitlab_6.8.1-omnibus.4-1_amd64.deb</a></td>
                     <td>e3696d1adc4872f067ed16b020ef55d2</td>