Point grc.bashrc to boxen

Dropping 'brew --prefix' for now, until Homebrew lets go of /usr/local.
Change shell to boxen/Homebrew's zsh

This is going to require boxen being set up first, obviously.
Going to have to keep that in mind for the future in the bootstrap scripts.
Add config.zsh

More detailed zsh config. Credit to @holman for the original.