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/build/config.js

@@ -25,7 +25,6 @@ const config = merge({
     watcher: !!argv.watch,
   },
   watch: [],
-  browsers: [],
 }, userConfig);
 
 module.exports = merge(config, {