Validate presence of 'poll_id' attribute on Poll::Question model

Fixes #1831

On branch aperez-validate-poll-question-is-selected
  Changes to be committed:
    modified:   app/models/poll/question.rb
    modified:   spec/models/poll/question_spec.rb
    modified:   spec/features/admin/poll/polls_spec.rb

app/models/poll/question.rb

@@ -20,6 +20,7 @@ class Poll::Question < ActiveRecord::Base
 
   validates :title, presence: true
   validates :author, presence: true
+  validates :poll_id, presence: true
 
   validates :title, length: { minimum: 4 }
   validates :description, length: { maximum: Poll::Question.description_max_length }