Grunt + Sass + Autoprefixer

Gruntfile.js

module.exports = function(grunt) {
  grunt.initConfig({
    pkg: grunt.file.readJSON('package.json'),
    sass: {
      dist: {
        options:{
          style:'compressed'
        },
        files: {
          'css/style.css' : 'scss/style.scss'
        }
      }
    },
    autoprefixer:{
      dist:{
        files:{
          'css/style.css':'css/style.css'
        }
      }
    },
    watch: {
      css: {
        files: 'scss/*.scss',
        tasks: ['sass', 'autoprefixer']
      }
    }
  });
  grunt.loadNpmTasks('grunt-contrib-sass');
  grunt.loadNpmTasks('grunt-contrib-watch');
  grunt.loadNpmTasks('grunt-autoprefixer');
  grunt.registerTask('default',['watch']);
}

package.json

{
  "name": "jshawl.com",
  "version": "0.1.0",
  "devDependencies": {
    "grunt": "~0.4.1",
    "grunt-contrib-sass": "~0.3.0",
    "grunt-contrib-watch": "~0.4.4",
    "grunt-autoprefixer": "~0.2.20130806"
  }
}

Thanks to this gist a few years ago I finally grasped enough to get my sweet Grunt setup running. Thank you!
Here's my updated version for 2018:
https://gist.github.com/squarecandy/a27671c96f3d226e0d4b9a118bfd1314

• using grunt-sass and node-sass to avoid ruby dependency
• using "plain" autoprefixer as suggested above
• spits out valid source maps, which can be a challenge to get working with a sass > autoprefixer pipeline...