Invoke prepack after bundle has been generated.

This plugin invokes `prepack` using the `transform()` method. But it shouldn't be this way. It should be `transformBundle()` because `prepack` should be invoked after `rollup` has created the bundle.

Look [here](https://github.com/TrySound/rollup-plugin-uglify/blob/master/index.js) for how it's done for the `uglify` plugin. 