Filter out no_release roles, as they don't run the deploy flow tasks, an...

...d thus are missing BRANCH, revisions.log, etc.

Basically, the issue here is that Capistrano allows you to define custom roles that don't participate in the release process, e.g. a :redis role or similar. You do this by defining your custom role with the :no_release attribute, e.g.
```
role :redis, :no_release => true
```

This fix uses the built in 'release_roles' DSL method which filters out hosts with the :no_release option set.

The impact here is that currently, the change tracking functionality is broken in that flowdock tries to write to a directory that doesn't exist, which causes the task to fail.

Thanks,

Andy