In dot, only set ZSH if not already set, install brew before updating

New to getting my dotfiles into github, new to zsh (haven't even switched yet), I just forked your dotfiles repo and started hacking on it. Please excuse me if this pull request doesn't make any sense

Two changes to the `dot` script: first call the `install.sh` script and only then call `brew update`, and setting the `ZSH` variable differently.