JPG image optimization not working
https://github.com/gruntjs/grunt-contrib-imagemin/issues/16
npm uninstall grunt-contrib-imagemin && npm install grunt-contrib-imagemin
node-jpegtran-bin
https://github.com/yeoman/node-jpegtran-bin
npm install --save jpegtran-bin
换个路径好了，把路径下的其他图片删掉也好了