rickvanlieshout.com/gatsby-node.js

var rucksack = require('rucksack-css')
var lost = require("lost")
var cssnext = require("postcss-cssnext")

exports.modifyWebpackConfig = function(config, env) {
    config.merge({
        postcss: [
            lost(),
            rucksack(),
            cssnext({
                browsers: ['>1%', 'last 2 versions']
            })
        ]
    })

    return config
};