Outdated reference to CanCan in README

There is a reference to https://github.com/ryanb/cancan in README file:
https://github.com/plataformatec/devise/blob/master/README.md#configuring-multiple-models

CanCan is no longer maintained. I think README should link to https://github.com/CanCanCommunity/cancancan or suggest using https://github.com/elabs/pundit