Use Pillow instead of ImageMagick

Some [arcticles](https://blog.uploadcare.com/the-fastest-production-ready-image-resize-out-there-part-0-7c974d520ad9) say that [Pillow](http://python-pillow.org/) fork called [Pillow-SIMD](https://github.com/uploadcare/pillow-simd) can be much faster than ImageMagick. As we are often in need to process many big-sized screenshots, maybe we can give Pillow a try?