use homebrew/opt instead of homebrew/Library/LinkedKegs

You know how `icu4c` appears to install itself every time you run? That's because it's an unlinked keg and we don't look for it in the right place.

Using `/opt/boxen/homebrew/opt` instead of `/opt/boxen/homebrew/Library/LinkedKegs` fixes this. `opt` contains symlinks to all installed kegs, instead of just the linked ones.