Update webpack.merge imports

Signed-off-by: Julius Härtl <jus@bitgrid.net>

webpack.dev.js

@@ -1,4 +1,4 @@
-const merge = require('webpack-merge');
+const { merge } = require('webpack-merge');
 const common = require('./webpack.common.js');
 
 module.exports = merge(common, {

webpack.prod.js

@@ -1,4 +1,4 @@
-const merge = require('webpack-merge')
+const { merge } = require('webpack-merge')
 const common = require('./webpack.common.js')
 
 module.exports = merge(common, {