Remove Rails 5 deprecation warning.

db/migrate/20120809174649_create_post_actions.rb

@@ -5,7 +5,7 @@ class CreatePostActions < ActiveRecord::Migration
       t.integer :user_id, null: false
       t.integer :post_action_type_id, null: false
       t.datetime :deleted_at
-      t.timestamps
+      t.timestamps null: false
     end
 
     add_index :post_actions, ["post_id"]