chore(package): update autoprefixer to version 9.4.0

Author: greenkeeper[bot]

package.json

@@ -25,7 +25,7 @@
     "url": "https://github.com/textpattern/textpattern-default-theme/issues"
   },
   "devDependencies": {
-    "autoprefixer": "9.3.1",
+    "autoprefixer": "9.4.0",
     "clean-webpack-plugin": "1.0.0",
     "copy-webpack-plugin": "4.6.0",
     "cross-env": "5.2.0",