Add upgrade to chef_certgen NIF
    
Without an upgrade function, we see the following error report
when

running the unit tests:
    
    =ERROR REPORT==== 11-Mar-2013::10:22:45 ===
    The on_load function for module chef_certgen returned
    {error,
     {upgrade, "Upgrade not supported by this NIF library."}}
    
Adding upgrade that is identical to load resolves it