add postinstall

Author: Marco Garcia

package.json

@@ -27,7 +27,7 @@
     "test": "nyc mocha",
     "prepare": "npm run build",
     "docs": "typedoc src/gen/api",
-    "prepack": "npm run build"
+    "postinstall": "npm run build"
   },
   "nyc": {
     "include": [