fix firebase version to 2.4.2 (angular#192)

Otherwise, firebase 3.x will be installed, which is not supported yet.

Author: rexebin

docs/1-install-and-setup.md

@@ -16,7 +16,7 @@ The Angular CLI's `new` command will set up the latest Angular build in a new pr
 ### 2. Install AngularFire 2 and Firebase
 
 ```bash
-npm install angularfire2 firebase --save
+npm install angularfire2 firebase@2.4.2 --save
 ```
 
 Now that you have a new project setup, install AngularFire 2 and Firebase from npm.