bump grunt-autoprefixer to ~2.1.0

package.json

@@ -34,7 +34,7 @@
     "btoa": "~1.1.2",
     "glob": "~4.3.1",
     "grunt": "~0.4.5",
-    "grunt-autoprefixer": "~2.0.0",
+    "grunt-autoprefixer": "~2.1.0",
     "grunt-banner": "~0.2.3",
     "grunt-contrib-clean": "~0.6.0",
     "grunt-contrib-compress": "~0.13.0",