Fix default gravity in MiniMagick resize_and_pad helper

The default gravity option in the MiniMagick `resize_and_pad` helper is relying on an RMagick constant.  This was causing an error in my uploader (`NameError: uninitialized constant Magick`) when I wasn't passing in a value for the gravity.  Sample class that was giving me the issue:

```ruby
class ImageUploader < CarrierWave::Uploader::Base
  include CarrierWave::MiniMagick

  version :thumbnail do
    resize_and_pad(160, 160)
  end
end
```
This probably won't affect many people since the docs specify using `process` directly.