Enhance compatibility to Magento v1.4.0.2

I know this module is only supported from 1.5.0.0 onwards. But it works quite well except for the "Check System" page where a fatal PHP error occurs since the method `getEdition` is not available:

```
Fatal error: Call to undefined method Mage::getEdition() in /path/to/magento/app/code/community/FireGento/Debug/Block/Diagnostic/CheckSystem.php on line 67
```

There are probably some ways of determining the correct edition by looking at some files but in order to render the page this fix should do the trick just nicely.