package.json: Fix non-standard license property.
package.json: Fix non-standard license property.

simohaitham/grunt-uncss,kingsj/grunt-uncss,sparanoid/grunt-uncss-inline,tazminweb/grunt-uncss,kingsj/grunt-uncss,simohaitham/grunt-uncss,sparanoid/grunt-uncss-inline,addyosmani/grunt-uncss,addyosmani/grunt-uncss,tazminweb/grunt-uncss