FIX: Added in required PHP constant value 'CDN_OFFER_URL

FIX: Added in required PHP constant value 'CDN_OFFER_URL' which was missing from the installer config causing the installation to complete but display a notice on environments where error reporting is enabled.

![screen shot 2014-04-25 at 15 09 27](https://cloud.githubusercontent.com/assets/511641/2801502/470c997e-cc83-11e3-9ac4-0e3a59eabe54.png)