Upgrade PostCSS build deps (#681)

* Move supported browser list to package.json.

That way all build tools can use it.

* Specify supported browsers explicitly.

This is necessary for the latest version of browserslist/autoprefixer.

This also drops IE 10 support, as it was accidental anyway.

* Upgrade PostCSS build deps.

postcss.config.js

@@ -4,7 +4,6 @@ module.exports = {
   plugins: [
     require('postcss-import')({ addDependencyTo: webpack }),
     require('postcss-cssnext')({
-      browsers: ['last 2 versions', 'IE > 10'],
       features: {
         customProperties: {
           variables: {