fixed changed location of stage3 on linux servers

Gentoo changed the urls of the stage3 on their servers, it's now for
example:

http://distfiles.gentoo.org/releases/amd64/autobuilds/current-stage3-amd64/stage3-amd64-20140619.tar.bz2
fix in order to explicitly use debian squeeze install cd

the generation script doesn't work with wheezy install-cd
it's a temporary fix
Merge branch 'master' of https://github.com/kakwa/genautoo