fix(package): update autoprefixer to version 9.4.6

Closes #73

Author: greenkeeper[bot]

package.json

@@ -8,7 +8,7 @@
   },
   "bugs": "https://github.com/static-dev/spike-css-standards/issues",
   "dependencies": {
-    "autoprefixer": "9.3.1",
+    "autoprefixer": "9.4.6",
     "cssnano": "4.1.7",
     "postcss-attribute-case-insensitive": "4.0.0",
     "postcss-calc": "7.0.1",