Compatibility downgrade of "grunt-contrib-imagemin"

Same problem here: https://github.com/yeoman/generator-webapp/issues/140

I'm always getting those dialog windows. That's a bit too much "dependency" for me...

![screen shot 2013-08-24 at 10 31 38 pm](https://f.cloud.github.com/assets/1567498/1021389/b0914044-0cfc-11e3-88d3-14ac49ecebff.png)