Revert `gh` to `hub`

`hub`, like `gh`, is now powered by `go`.

https://github.com/github/hub#2x

Since `gh` is a GitHub CLI, why not use the CLI created by GitHub themselves.

Additionally, `hub` handles `$ git clone thoughtbot/laptop` out of the box.
Most of our `brew_install_or_upgrade`s use the version numbers instead of `HEAD`. Is there a reason to use `HEAD` here?