Update Training logo links to main site

All GitHub Training logos now link to the https://training.github.com home page.

![screen shot 2014-03-17 at 2 37 26 pm](https://f.cloud.github.com/assets/352082/2441393/0f8cce7a-ae14-11e3-81e3-843863799103.png)
![screen shot 2014-03-17 at 2 37 39 pm](https://f.cloud.github.com/assets/352082/2441394/0f8d15c4-ae14-11e3-8009-2f0039ad11be.png)

/cc @matthewmccullough