add postinstall to build dist

 - otherwise we cannot use it directly as a git package

Author: Jackman3005

package.json

@@ -5,6 +5,7 @@
   "main": "dist/lib/promise-poller.js",
   "scripts": {
     "prepublishOnly": "npm run build",
+    "postinstall": "npm run build",
     "build": "babel -d dist src",
     "clean": "rm -rf dist",
     "lint": "eslint src",