fix review request constraint

spec/factories/review_requests.rb

@@ -3,7 +3,7 @@
 # Table name: review_requests
 #
 #  id          :bigint           not null, primary key
-#  answered    :string
+#  answered    :string           not null
 #  created_at  :datetime         not null
 #  updated_at  :datetime         not null
 #  question_id :bigint           not null

spec/models/review_request_spec.rb

@@ -3,7 +3,7 @@
 # Table name: review_requests
 #
 #  id          :bigint           not null, primary key
-#  answered    :string
+#  answered    :string           not null
 #  created_at  :datetime         not null
 #  updated_at  :datetime         not null
 #  question_id :bigint           not null