Remove support for EOL'd ubuntu-13.10 (saucy)

Saucy reached its EOL on 7/17/14.

https://lists.ubuntu.com/archives/ubuntu-announce/2014-June/000185.html
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Rely on logic of `brew outdated` to decide whether a brew is upgradable.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
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
Clean up shell style