Update rubocop

[difernandez]

In october Rubocop finally released v1.0, their first stable version 🎉 :

The big news for end users and maintainers of extensions is that we’re finally embracing fully Semantic Versioning, which should make the upgrade process simpler (painless?) for everyone. Going forward the following things will happen only on major releases:

• enabling of new cops
• changes to the default cop configuration
• breaking API changes

This means that we should be able to update and safely lock new projects to the major instead of being limited in the minor. This means updating rules that may have changed their name before 1.0 and after 0.82 (version we are currently using) if needed and take a look at new rules enabled in the same time frame.

This should update potassium as well as projects generated with it.