Migrate to webpack 4

webpack.config.main.js

@@ -1,6 +1,8 @@
 /* eslint-disable import/no-commonjs */
 'use strict';
 
+const path = require('path');
+
 const merge = require('webpack-merge');
 
 const base = require('./webpack.config.base');
@@ -8,7 +10,8 @@ const base = require('./webpack.config.base');
 module.exports = merge(base, {
   entry: './src/main.js',
   output: {
-    filename: './src/[name]_bundle.js',
+    path: path.join(__dirname, 'src'),
+    filename: '[name]_bundle.js',
   },
   node: {
     __filename: true,