chore(package): update autoprefixer to version 9.7.4

Author: greenkeeper[bot]

package.json

@@ -17,7 +17,7 @@
     "url": "https://github.com/textpattern/textpattern-jquery-ui-theme/issues"
   },
   "devDependencies": {
-    "autoprefixer": "9.7.3",
+    "autoprefixer": "9.7.4",
     "clean-webpack-plugin": "3.0.0",
     "cross-env": "6.0.3",
     "css-loader": "3.4.0",