I'll trying to use uglify with webpack but got this error: 

```
Invalid configuration object. Webpack has been initialised using a configuration object that does not match the API schema.
 - configuration has an unknown property 'optimization'. These properties are valid:
   object { amd?, bail?, cache?, context?, dependencies?, devServer?, devtool?, entry, externals?, loader?, module?, name?, node?, output?, performance?, plugins?, profile?, recordsInputPath?, recordsOutputPath?, recordsPath?, resolve?, resolveLoader?, stats?, target?, watch?, watchOptions? }

```

I have 

```
const UglifyJsPlugin = require('uglifyjs-webpack-plugin');

```

and 

```
optimization: {
        minimizer: [new UglifyJsPlugin()]
    }

```

in my webpack.config.js, looks like webpack is not recognizing 'optimization'?