[#79780042] Upgrade Thor to clear Gem logjam

[Pivotal ticket](79780042-upgrade-thor)

On Backoffice, upgrades to other dev tools like Foreman, Rubocop and Pronto are being blocked by our current version of Mulder pinning Thor to '~> 0.18.1' when all those tools have dependency chains involving Thor 0.19.x

Upgrade the development dependency for Mulder to use Thor 0.19, the current version.