Move to postcss/css-next

[banderson]

From the npm install output:

npm WARN deprecated cssnext-loader@1.0.1: cssnext is now postcss-cssnext. cssnext-loader is deprecated. Use postcss-loader. See postcss-cssnext migration guide http://cssnext.io/postcss/
npm WARN deprecated cssnext@1.8.4: cssnext is now postcss-cssnext. cssnext is deprecated. See postcss-cssnext migration guide http://cssnext.io/postcss/

Migration guide available here.