Fix #2

```ruby
class CatCell  < Cell::Rails
  include Cell::Rails::Capture
  
  def show
  end
end

class DogCell  < Cell::Rails
  def show
  end
end
```
```
#index.html.slim
= render_cell "cat"
= render_cell "dog"
```

In this situation, an error occurs because `DogCell` does not have `global_tpl` method