Attempt to resolve duplicate webhook calls

For me, when I pushed multiple statuses, I'd get duplicate messages in slack for the second append:

```
c['status'].append(slack.SlackStatusPush("subdomain", "key", "#channel1", "localhost", "project1"))
c['status'].append(slack.SlackStatusPush("subdomain", "key", "#channel2", "localhost", "project2"))
```

When project2 build completed, I'd get a message in both #channel1 and #channel2. For me, the changes in this PR resolved the problem.