Modify configuration to match new version of webpacker

Error was:
```
ERROR in chunk admin [entry]
js/[name]-[contenthash].js
Cannot use [chunkhash] or [contenthash] for chunk in 'js/[name]-[contenthash].js' (use [hash] instead)
```

config/webpack/development.js

@@ -2,4 +2,7 @@ process.env.NODE_ENV = process.env.NODE_ENV || 'development'
 
 const environment = require('./environment')
 
+const config = environment.toWebpackConfig();
+config.output.filename = "js/[name]-[hash].js";
+
 module.exports = environment.toWebpackConfig()