Use `gh`, not `hub`

[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]: github/hub#475

Matches thoughtbot/laptop:

https://github.com/thoughtbot/laptop/commit/d9a9dfe09bf5898172c752c87be7c5ae6537c5f2