Use Devise 3.4.0 in test environment

The Devise update did break the Cucumber features suite:

Minor fix misplaced requirement in dummy app
Devise must be required after Rails.

Minor fix Cucumber scenarios after Devise update.
The scaffolded controllers use `respond_with` but did not explicitely
define which formaats they `respond_to`. That used to cause no issues
but now triggers an error, possibly because of the new dependency of
Devise to the 'responders' gem (which was extracted from Rails, see
the Devise 3.4.0 changelog).
These errors have no interest in the context of the scenarios in which
they occur, that's why I opted for silencing them.