Fix #4115: Upgrade to webpack 4.x.

https://github.com/rails/webpacker/blob/master/docs/v4-upgrade.md

postcss.config.js

@@ -0,0 +1,12 @@
+module.exports = {
+  plugins: [
+    require('postcss-import'),
+    require('postcss-flexbugs-fixes'),
+    require('postcss-preset-env')({
+      autoprefixer: {
+        flexbox: 'no-2009'
+      },
+      stage: 3
+    })
+  ]
+}