Fix broken specs

spec/factories/conferences.rb

@@ -3,7 +3,7 @@
 FactoryBot.define do
   factory :conference do
     sequence(:title) { |n| "Conference-#{n}" }
-    email
+    email { 'foo@example.com' }
     description { 'MyText' }
     start_date { '2014-07-29 21:29:13' }
     end_date { '2014-07-31 21:29:13' }

spec/models/call_for_participation_spec.rb

@@ -1,7 +1,8 @@
 require 'rails_helper'
 
 RSpec.describe CallForParticipation, type: :model do
-  let(:cfp) { build :call_for_participation }
+  let(:conference) { create :conference }
+  subject(:cfp) { build :call_for_participation, conference_id: conference.id }
 
   it 'belongs to a conference' do
     conference = create :conference