graphite source uri changed

Graphite project has been moved from Launchpad to Github. 
Version 0.9.10 is available at both locations, but 0.9.11 and newer (0.9.12, to the moment) can be found at Github only. This can be easily fixed by just overriding attribute node['graphite']..['uri'] in role, but changing a cookbook is better because it avoids unnecessary role complexity.