Upgraded webpack PostCSS plugin definition

[danielduan]

Issue: #1061 No PostCSS config found

What I did

It seems like either the config is defined differently in Webpack 2 or we upgraded to a newer version of PostCSS. PostCSS can't read our current plugin definition so I upgraded it to the latest listed on the docs.

How to test

1. create a story, then create a css file for the story.
2. put some test styles in the css.
3. in the story, either import 'styles.css', or require('styles.css').

it should not throw an error during build and console log the style-loader contents.

[ndelangen]

First merge! 🍾