Resolving issue with set_current_revision not declared

The deployment fails with 
```
cap aborted!
Don't know how to build task 'rsync:set_current_revision'

Tasks: TOP => deploy:set_current_revision
(See full trace by running task with --trace)
The deploy has failed with an error: #<RuntimeError: Don't know how to build task 'rsync:set_current_revision'>
```

The solution is to create the task as described in #15.