Fix a few broken links

Noticed one link was broken, so I did a sweep and fixed a few more. 

There are also two broken links (on index.html) for which I couldn't find the original content. Looks like the blogs that hosted them were completely removed, although the main sites that hosted the blogs are still around:

* http://lambdaphant.com/blog/interview-with-tom-preston-werner-from-github (main site http://lambdaphant.com/ is still alive)
* http://joshuaink2006.johnoxton.co.uk/blog/240/seven-and-a-half-questions-for-tom-werner (main site http://johnoxton.co.uk/ is still alive)

Cheers!