Use be_successful instead of be_success

Rails 5.2 warns:

> The `success?` predicate is deprecated and will be removed in Rails 6.0.
> Please use `successful?` as provided by Rack::Response::Helpers.

RSpec correspondingly provides `be_successful`.

spec/controllers/admin/conferences_controller_spec.rb

@@ -242,7 +242,7 @@ describe Admin::ConferencesController do
         it 're-renders the new template' do
           post :create, params: { conference:
                                               attributes_for(:conference, short_title: nil, organization_id: organization.id) }
-          expect(response).to be_success
+          expect(response).to be_successful
         end
       end
 
@@ -259,7 +259,7 @@ describe Admin::ConferencesController do
         it 're-renders the new template' do
           conference
           post :create, params: { conference: attributes_for(:conference, short_title: conference.short_title, organization_id: organization.id) }
-          expect(response).to be_success
+          expect(response).to be_successful
         end
       end
     end

spec/controllers/admin/event_schedules_controller_spec.rb

@@ -33,7 +33,7 @@ describe Admin::EventSchedulesController do
 
         it 'has 200 status code' do
           create_action
-          expect(response).to be_success
+          expect(response).to be_successful
         end
       end
 
@@ -80,7 +80,7 @@ describe Admin::EventSchedulesController do
         end
 
         it 'has 200 status code' do
-          expect(response).to be_success
+          expect(response).to be_successful
         end
       end
 
@@ -115,7 +115,7 @@ describe Admin::EventSchedulesController do
 
       it 'has 200 status code' do
         destroy_action
-        expect(response).to be_success
+        expect(response).to be_successful
       end
     end
   end

spec/controllers/admin/registration_periods_controller_spec.rb

@@ -104,7 +104,7 @@ describe Admin::RegistrationPeriodsController do
                                                 start_date: nil,
                                                 end_date:   nil)
           }
-          expect(response).to be_success
+          expect(response).to be_successful
         end
       end
     end