chore(package): update autoprefixer to version 8.4.0

Closes #33

Author: greenkeeper[bot]

package.json

@@ -59,7 +59,7 @@
     "postcss-load-config": "1.2.0"
   },
   "devDependencies": {
-    "autoprefixer": "7.2.6",
+    "autoprefixer": "8.4.0",
     "ava": "0.25.0",
     "coveralls": "2.13.2",
     "execa": "0.9.0",