Make Rubocop happy

Make Rubocop happy

diff --git a/db/migrate/20190625085735_add_vote_count_index_to_topic_custom_fields.rb b/db/migrate/20190625085735_add_vote_count_index_to_topic_custom_fields.rb
index 272906b..bc49f1d 100644
--- a/db/migrate/20190625085735_add_vote_count_index_to_topic_custom_fields.rb
+++ b/db/migrate/20190625085735_add_vote_count_index_to_topic_custom_fields.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class AddVoteCountIndexToTopicCustomFields < ActiveRecord::Migration[5.2]
   def change
     execute <<~SQL

GitHub sha: 184478b8