Revert "use dev plugin"

This reverts commit 6a95ad5.

Author: youfoundron

example/gatsby-config.js

@@ -1,6 +1,5 @@
 module.exports = {
   plugins: [
-    'develop-purify-css'
-    // 'gatsby-plugin-purify-css'
+    'gatsby-plugin-purify-css'
   ]
 }