suggest rake -f in Contributing README section

Changes this:
![image](https://cloud.githubusercontent.com/assets/21379/4633593/c0afe11a-53c8-11e4-8e51-cd4ebaa045a2.png)


To this:
![image](https://cloud.githubusercontent.com/assets/21379/4633613/db48e440-53c8-11e4-95b0-153d9c3684bd.png)