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.
Merge branch 'develop' of https://github.com/riker09/firegento-debug into develop