Rails 5 support CURRENT_TIMESTAMP

rails5 現在 support CURRENT_TIMESTAMP 成為 default value 的寫法，但 annotate_model 的 gem 好像不會自動偵測到

```ruby

class CreatePosts < ActiveRecord::Migration[5.0]
  def change
    create_table :posts do |t|
      t.datetime :modified_at, default: -> { 'CURRENT_TIMESTAMP' }
      t.timestamps
    end
  end 
end

```

https://stackoverflow.com/questions/1580805/how-to-set-the-default-value-for-a-datetime-column-in-migration-script


https://github.com/rails/rails/pull/20005/files