chore(package): update autoprefixer to version 9.7.4

Author: greenkeeper[bot]

package.json

@@ -10,7 +10,7 @@
     "url": "https://github.com/textpattern/textpattern-com-website/issues"
   },
   "devDependencies": {
-    "autoprefixer": "9.7.3",
+    "autoprefixer": "9.7.4",
     "cssnano": "4.1.10",
     "grunt": "1.0.4",
     "grunt-cli": "1.3.2",