Ensure required validations accepts subclasses

Allow users to create a custom validator that inherits from the acceptable paperclip validators, and will prevent an error from being raised in that case.

```
class MyValidator < Paperclip::Validators::AttachmentContentTypeValidator
end

class MyModel < ActiveRecord::Base
  has_attached_file :attachment
  validates_with MyValidator, attributes: :attachment, content_type: 'image/jpeg'
end
```