Fix PostCSS config.

Changes in syntax are documented in NextJS canary release.
See https://github.com/zeit/next.js/pull/10119/files

Author: emilbryggare

examples/nextjs/postcss.config.js

@@ -1,13 +1,15 @@
-const purgecss = require('@fullhuman/postcss-purgecss')({
-  content: ['./components/**/*.js', './pages/**/*.js'],
-  defaultExtractor: content => content.match(/[\w-/:]+(?<!:)/g) || [],
-})
-
+const purgecss = [
+  "@fullhuman/postcss-purgecss",
+  {
+    content: ["./components/**/*.js", "./pages/**/*.js"],
+    defaultExtractor: content => content.match(/[\w-/:]+(?<!:)/g) || []
+  }
+];
 module.exports = {
   plugins: [
-    require('postcss-import'),
-    require('tailwindcss'),
-    require('autoprefixer'),
-    ...(process.env.NODE_ENV === 'production' ? [purgecss] : []),
-  ],
-}
+    "postcss-import",
+    "tailwindcss",
+    "autoprefixer",
+    ...(process.env.NODE_ENV === "production" ? [purgecss] : [])
+  ]
+};