Node6 does not support syntax of trailing comma

Hi, when use this plugin, I get the following error.
```
/node_modules/add-asset-html-webpack-plugin/lib/index.js:121
      );
      ^

SyntaxError: Unexpected token )
```
My node version is v6.16.0, and the engines node of this plugin is greater or equal to 6. But node6 does not support trailing comma syntax.

I upgrade node to v8.x.x, this plugin work well.


