Update autoprefixer map option

Accommodate the grunt-autoprefixer 0.8.0 options refactor
by passing an object (instead of string) to map option
a la nDmitry/grunt-autoprefixer@c658fb7
and nDmitry/grunt-autoprefixer@1b7af91

Gruntfile.js

@@ -83,7 +83,9 @@ module.exports = function(grunt) {
       },
       dev: {
         options: {
-          map: 'assets/css/'
+          map: {
+            prev: 'assets/css/'
+          }
         },
         src: 'assets/css/main.css'
       },

package.json

@@ -25,7 +25,7 @@
   "devDependencies": {
     "bower": "~1.3.5",
     "grunt": "~0.4.5",
-    "grunt-autoprefixer": "~0.8.0",
+    "grunt-autoprefixer": "~0.8.1",
     "grunt-contrib-concat": "~0.4.0",
     "grunt-contrib-jshint": "~0.10.0",
     "grunt-contrib-less": "~0.11.3",