Remove migrations warning

DEPRECATION WARNING: Directly inheriting from ActiveRecord::Migration
is deprecated. Please specify the Rails release the migration was
written for:

class MigrationClass < ActiveRecord::Migration[4.2]

(called from require at bin/rails:4)

db/migrate/20171004025903_create_poll_question_answers.rb

@@ -1,4 +1,4 @@
-class CreatePollQuestionAnswers < ActiveRecord::Migration
+class CreatePollQuestionAnswers < ActiveRecord::Migration[4.2]
   def change
     create_table :poll_question_answers do |t|
       t.string :title