Silence deprecation warning from Devise

Warning was:
```
DEPRECATION WARNING: [Devise] including `Devise::TestHelpers` is deprecated and will be removed from Devise.
For controller tests, please include `Devise::Test::ControllerHelpers` instead.
```

spec/spec_helper.rb

@@ -150,7 +150,7 @@ def main_app
 
   config.include Shoulda::Matchers::Independent
 
-  config.include Devise::TestHelpers, type: :controller
+  config.include Devise::Test::ControllerHelpers, type: :controller
   config.include EngineRoutes, type: :controller
 
   config.include Warden::Test::Helpers, type: :feature

sufia.gemspec

@@ -31,6 +31,7 @@ Gem::Specification.new do |spec|
   spec.add_dependency 'select2-rails', '~> 3.5.9'
   spec.add_dependency 'json-schema'
   spec.add_dependency 'oauth'
+  spec.add_dependency 'devise', '~> 4.2'
   spec.add_dependency 'nest', '~> 1.1'
   spec.add_dependency 'mailboxer', '~> 0.12'
   spec.add_dependency 'acts_as_follower', '>= 0.1.1', '< 0.3'