Use `execute` instead of `exec_sql` in migration.

db/migrate/20170505035229_migrate_mailing_list_daily_updates_users_to_daily_summary.rb

@@ -2,7 +2,7 @@ class MigrateMailingListDailyUpdatesUsersToDailySummary < ActiveRecord::Migratio
   def change
     change_column_default :user_options, :mailing_list_mode_frequency, 1
 
-    UserOption.exec_sql(<<~SQL)
+    execute <<~SQL
     UPDATE user_options
     SET digest_after_minutes = 1440, email_digests = 't', mailing_list_mode = 'f'
     WHERE mailing_list_mode_frequency = 0 AND mailing_list_mode