Apply automatic rubocop fixes

db/migrate/20150421074734_add_file_index_to_files.rb

@@ -1,5 +1,7 @@
+# frozen_string_literal: true
+
 class AddFileIndexToFiles < ActiveRecord::Migration[4.2]
   def change
-    add_index(:files, [:context_id, :context_type])
+    add_index(:files, %i[context_id context_type])
   end
 end