homebrew: Replacement for `brew bundle`

Apparently `brew bundle` is deprecated and might, maybe be replaced in
the future. Until then we’ll run with this little thingy.

Disable Caskfile in `post-up` hook because I have quite a lot of apps
that haven’t been installed through `cask`, and I really can’t bother
with it right now.
rcm: Clean up `post-up` hook & make it executable

Apparently it has to be executable to even run when issuing `rcup`. The
more you know, eh?