npm install (#338)

Install dependencies with `npm install` before trying to run the demo.

Author: pedrocatre

demo/README.md

@@ -20,7 +20,9 @@ Run:
 
 ```bash
 git clone https://github.com/firebase/firebaseui-web.git
-cd firebaseui-web/demo
+cd firebaseui-web
+npm install
+cd demo
 ```
 
 This will clone the repository in the current directory.