Bumping zlib to 1.2.8 version and changing the URL from .bz2 to .gz for...

Bootstrap process fails due broken URL, at this time only .gz version is available. In addition to that, seems like the project is not using .bz2 anymore http://sourceforge.net/projects/libpng/files/zlib/1.2.8/

Anyway I can confirm that I'm able to boot with this version of zlib