Update task hooks

Change task hooks. Current ones could notify the event even if the
deploy/rollback fails.

Related: http://capistranorb.com/documentation/getting-started/flow/

- deploy:updated  -> deploy:finishing
- deploy:reverted -> deploy:finishing_callback
Merge pull request #20 from areina/update-hooks

Update task hooks
Fix post_graphite task

With only one parameter in block, it's the task. If you want to use
arguments, you need two parameters, task and args.
Less verbose way to call rake tasks