Replace blip.tv video links with YouTube counterparts

All the videos once hosted on Blip.tv have sailed away.

- Found what should be their versions hoisted up on YouTube
- Removed default playlist showing as that's gone as well
- Stubbed RailsConf 2008, not uploaded on YouTube account, but it [can be found here on Vimeo until then](http://vimeo.com/1099027)

A follow on from [replacing them on Learn:GitHub](https://github.com/github/learn.github.com/pull/27)