Update Autoprefixer and standardize browserlist location (#1899)

This updates Autoprefixer to 7.1.1, and also moves the browser target
list to package.json, in line with the recommendation made in this
article (https://evilmartians.com/chronicles/autoprefixer-7-browserslist-2-released)
from the developers of Autoprefixer and Browserslist.

resources/assets/config.json

@@ -14,10 +14,5 @@
   "cacheBusting": "[name]_[hash:8]",
   "watch": [
     "{app,resources/views}/**/*.php"
-  ],
-  "browsers": [
-    "last 2 versions",
-    "android 4",
-    "opera 12"
   ]
 }