Losslessly compress public/images using image_optim gem

I noticed that some images were loading slower than expected (I know, I know, that's kinda weird) on git-scm.com. I ran [PageSpeed on git-scm.com](https://developers.google.com/speed/pagespeed/insights#url=http_3A_2F_2Fgit-scm.com), which confirmed that some images are not losslessly compressed. I compressed images using [image_optim gem](https://github.com/toy/image_optim), but there are certainly [other ways](https://github.com/botandrose/sprockets-image_compressor) to achieve a similar result. [Other](https://developers.google.com/speed/pagespeed/insights#url=mac.github.com) [GitHub websites](https://developers.google.com/speed/pagespeed/insights#url=windows.github.com) also contain non-optimized images.