fix build commonjs config

build/cooking.common.js

@@ -9,7 +9,7 @@ cooking.set({
   extends: ['vue2'],
   minimize: false,
   alias: config.alias,
-  externals: { vue: 'vue' }
+  externals: config.externals
 });
 
 cooking.add('output.filename', 'element-ui.common.js');