Update rubocop to version 0.51.0
Update rubocop to version 0.51.0

Summary
-------

Apparently there's a low severity security vulnerability in version
range < 0.48.1 - or so sayeth the Github. At any rate, this is part of
a larger effort to update the development dependencies of Futurist.

indiebrain/futurist,indiebrain/futurist