Missing zlib and yaml

[mlamoore]

I'm running summon-arm-toolchain on a fresh Ubuntu 12.10 install. I already ran apt-get on the packages recommended in the Linux section of the readme (except that build-dep doesn't exist on Ubuntu).

I got some errors while running the script, and to get it to compile I had to install packages zlib1g-dev and python-yaml. As I tried a few other yaml packages before I tried the Python one, I think it would be helpful if these packages were added to the list of packages on the readme.