Apply automatic rubocop fixes

db/migrate/20180222145909_fix_timestamps_on_feedback.rb

@@ -1,10 +1,10 @@
+# frozen_string_literal: true
+
 class FixTimestampsOnFeedback < ActiveRecord::Migration[4.2]
   def up
     change_column_default(:user_exercise_feedbacks, :created_at, nil)
     change_column_default(:user_exercise_feedbacks, :updated_at, nil)
   end
 
-  def down
-    
-  end
+  def down; end
 end