Update README.md.
Update README.md.

* switch to the shields.io theme for the dependency badges
* use the link to the our comparison page
* use the proper case for Node.js and Grunt

tomByrer/clean-css,island205/clean-css,vlajos/clean-css,yomed/clean-css,stuht/clean-css,GerHobbelt/clean-css,MartyIX/clean-css,scottgonzalez/clean-css,jakubpawlowicz/clean-css,shelsonjava/clean-css,webconnex/clean-css,HossainKhademian/CleanCss