Bug fix for `image` being overwritten from VIPS::Image.new(current_path) in #resize_image

Previously, when resize_image was called, it would re-read the image from disk (adding the amount of shrink, and setting :sequential to true). This caused previous manipulations to be erased; this was especially problematic when trying to crop a photo using extract_area and then resize it to fill.

This may not be the best fix, but it works. I was unable to find a method to set :sequential to true in ruby-vips; perhaps, I just didnt look hard enough.

Please let me know if you need anything changed, or if I can prepare my commit better.

Best,
Seve