Make grunt-modernizr compatible to Grunt 0.4

In v0.4 `grunt.file.expandFiles()` is replaced with `grunt.file.expand({filter: 'isFile'}, …)`

See https://github.com/gruntjs/grunt/wiki/Upgrading-from-0.3-to-0.4 and https://github.com/gruntjs/grunt/wiki/grunt.file