chore(package): update autoprefixer to version 9.1.5

Author: greenkeeper[bot]

package.json

@@ -10,7 +10,7 @@
     "url": "https://github.com/textpattern/textpattern-com-website/issues"
   },
   "devDependencies": {
-    "autoprefixer": "9.1.4",
+    "autoprefixer": "9.1.5",
     "cssnano": "4.1.0",
     "dart-sass": "1.13.1",
     "fibers": "3.0.0",