Replaced rasterized content in svg file.

New svg computer images were obtained from;
http://commons.wikimedia.org/wiki/File:Computer.svg

New svg github logos were obtained from;
https://github.com/logos

The Computer and GitHub images are inlined in the svg. I attempted to
embed the images, but the embedded svgs would not render when I made the
site. github-collaboration.svg would render correctly when viewed by
itself, but not within the lesson page.

Also removed unnecessary odg.