make implicit explicit

package.json

@@ -6,7 +6,8 @@
   "author": "Guillaume Vincent <guillaume@oslab.fr>",
   "repository": "lesspass/frontend",
   "scripts": {
-    "build": "rm -rf dist && gulp && rm dist/index.html"
+    "build": "rm -rf dist && gulp && rm dist/index.html",
+    "start": "NODE_ENV=production node server.js"
   },
   "dependencies": {
     "express": "^4.14.0",