Fix default gravity in MiniMagick resize_and_pad helper

[abevoelker]

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:

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.

[bensie]

Good catch :) Thanks!