avoid destructuring for node < 6 compat

Author: yyx990803

lib/style-compiler/index.js

@@ -26,13 +26,13 @@ module.exports = function (css, map) {
   }
 
   // use the same config loading interface as postcss-loader
-  loadPostcssConfig(vueOptions.postcss).then(({ plugins, options }) => {
-    plugins = [trim].concat(plugins)
-    options = Object.assign({
+  loadPostcssConfig(vueOptions.postcss).then(config => {
+    var plugins = [trim].concat(config.plugins)
+    var options = Object.assign({
       to: this.resourcePath,
       from: this.resourcePath,
       map: false
-    }, options)
+    }, config.options)
 
     // add plugin for vue-loader scoped css rewrite
     if (query.scoped) {