Update postcss-load-config dependency to the latest version

[Yankovsky]

I've experienced problems with resolving postcss plugins defined in .postcssrc.json or in postcss.config.js. New version solved them.
My current working setup is .postcssrc.json file in the root directory:

{
  "plugins": {
    "postcss-nested": {},
    "autoprefixer": {}
  }
}

[dferber90]

Published as v6, thanks! https://github.com/dferber90/jest-transform-css/releases/tag/v6.0.0