Fixes error on Ruby 2.0.0/Rails 4.0.0.beta1

Fixes error on Ruby 2.0.0/Rails 4.0.0.beta1:

```
/Users/lupinglade/.rvm/gems/ruby-2.0.0-p0@rails-4.0.0.beta1/bundler/gems/active_admin-d2b9840832f4/lib/active_admin/reloader.rb:78:in `updated_at': wrong number of arguments (1 for 0) (ArgumentError)
```