Maintain transparent background in #resize_to_fill

When using this method on a PNG image, previously it would loose
transparency. This is because the "-extent" ImageMagick option sets a
background color. We should explicitly set the background color to
transparent, it seems to work for CarrierWave.

See https://github.com/refile/refile-mini_magick/issues/11.

lib/image_processing/mini_magick.rb

@@ -110,6 +110,7 @@ module ImageProcessing
           yield cmd if block_given?
           cmd.resize "#{width}x#{height}^"
           cmd.gravity gravity
+          cmd.background "rgba(255,255,255,0.0)" # transparent
           cmd.extent "#{width}x#{height}"
         end
       end