Change file order at migration

Hi,

Dir.glob lists files:

1. app/models/user/item.rb
2. app/models/user.rb

This commit:

1. app/models/user.rb
2. app/models/user/item.rb

Why need change order?

Migrate using original code failed wit special case association model.
because `String#constantize` doesn't load `app/model/user.rb` when processing `app/models/user/item.rb` in this case.

app/models/user.rb
```ruby
class User
  include Cequel::Record

  key :id, :text
  has_many :item, class_name: "User::Item"
end
```

app/models/user/item.rb
```ruby
class User
  class Item
    include Cequel::Record

    belongs_to :user
    key :id, :text
  end
end
```

thank you.