Remove link to buildr book on website

As discussed on dev list, (http://buildr.markmail.org/search/list:dev#query:list%3Adev+page:1+mid:nnui5zlvow2hdks6+state:results)

we have seen no real good come of having the buildr book on the website. It's quite outdated.

Additionally, we should look at creating the epub of the book ourselves and hosting it for free on the website. We should make sure the buildr pdf is up to date.