Make `brew_is_upgradable` mac function more robust

Rely on logic of `brew outdated` to decide whether a brew is upgradable.

Fixes #276 and #282:
* https://github.com/thoughtbot/laptop/issues/276
* https://github.com/thoughtbot/laptop/issues/282
Don't redirect stderr to /dev/null

`&>` redirects both `stderr` and `stdout`.
`>` redirects only stdout.

Use `&>` when we explicitly want to ignore/discard error messages,
which is rarely.

`command -v` outputs only stdout.
So, `>` is appropriate because there is expected `stderr` output.
In the rare case that command fails,
we want the reason printed.
Switch from `hub` to `gh`

[gh] is a [hub] reimplementation that's much faster and is now the official
Github CLI. It appears that "hub" is [deprecated].

[gh]: https://github.com/jingweno/gh
[hub]: https://github.com/github/hub
[deprecated]: https://github.com/github/hub/issues/475
Update Homebrew script URL to new location

https://github.com/thoughtbot/laptop/issues/291
Setting up our necessities for development.