Revert "Change let to var (#738)"

This reverts commit 31506f2.

Author: kazupon

lib/style-compiler/load-postcss-config.js

@@ -1,6 +1,6 @@
 var load = require('postcss-load-config')
 
-var loaded
+let loaded
 
 function isObject (val) {
   return val && typeof val === 'object'