chore : Adds prepare script

packages/medusa/package.json

@@ -31,6 +31,7 @@
   },
   "scripts": {
  "start": "nodemon --watch plugins/ --watch src/ --exec babel-node src/app.js",
+     "prepare": "cross-env NODE_ENV=production npm run build",
     "build": "babel src -d dist",
     "serve": "node dist/app.js",
     "test": "jest"