Upgrade Devise from 4.1.1 to 4.2.0.

This fixes an issue with Rails 5 and brings us up-to-date with the latest Devise release. It also deprecates `Devise::TestHelpers` in favor of `Devise::Test::ControllerHelpers`.

Changelog: https://github.com/plataformatec/devise/blob/v4.2.0/CHANGELOG.md#420---2016-07-01

Working toward #14286, as always.

See merge request !6461