Fix example setup

Otherwise, on a fresh clone, `npm install` from the example dir gives:
```
ERROR in ./src/messages.json
Module build failed: Error: Cannot find module 'loader-utils'
```

Author: cletusw

example/package.json

@@ -4,7 +4,7 @@
   "description": "Small example for messageformat-loader",
   "scripts": {
     "build": "webpack",
-    "postinstall": "cd .. && npm install --no-save messageformat",
+    "postinstall": "cd .. && npm install && npm install --no-save messageformat",
     "prepare": "npm run build"
   },
   "main": "src/index.js",