grunt-contrib-jshint ignores has no effect

I would like to exclude libs directory from being lint'ed. However, ignores in options and planted .jshintignore file in project directory won't make libs to be excluded.

 jshint: {
  options: {
    smarttabs: true,
    ignores: ['public/js/libs/**/*.js']
  },

  all: [
        'Gruntfile.js', 
        'public/js/**/*.js'
      ]

},

grunt version:

 grunt-cli v0.1.11
 grunt v0.4.2
 grunt-contrib-jshint@0.7.2

What did I miss out?