Use FastImage to get image size instead of mini_magick.
Use FastImage to get image size instead of mini_magick.

- Change require call and error message.
- Always detect format using File#extname.
- Rename #load_image to #get_image_size.
- Detect format using FastImage in #get_image_size.

cantino/ruby-readability,Orphist/ruby-readability,framgia/ruby-readability