Using grunt.util for newer grunt versions

`grunt.utils` was deprecated in favor of `grunt.util` in Grunt 0.4.0. See https://github.com/gruntjs/grunt/wiki/Upgrading-from-0.3-to-0.4 (last point under headline API).

With this change I was able to execute BonsaiJS tests against Saucelabs using Grunt 0.4.0.