Update `css` dependency

Thank you for the great tool!

Old css-parse has some weird bugs (e. g. at-rules parsing), so I've updated all expected test results according to css-parse and css-stringify changes (add semicolons, remove [empty rulesets](https://github.com/visionmedia/css-stringify/issues/7) in `empty.out.css`). Also I've added test script in `package.json` for Travis CI.

P. S. [grunt-spriter](https://github.com/nDmitry/grunt-spriter) =)