updated service hook so it is in sync with recent docker changes. 

We have recently made some changes to the docker hosted services, and these changes bring the service hook back into sync.

- Changed the url from ``index.docker.io`` to ``registry.hub.docker.com``
- Changed twitter handle from ``getdocker`` to ``docker``
- Changed URL to ``www.docker.com`` from ``www.docker.io``
- added a docker logo