Always use install instead of boxen-install.

boxen-install uses a pre-compiled version of a package instead of
building it for the current system. This saves on install time, but
these pre-compiled versions are built with newer versions of Mac, this
means that some packages don't work (tmux, emacs, etc). So to fix this
problem, we use brew install all of the time. This means that packages
are built for the current system, which is one of the points behind
homebrew.