Fixed empty brace problems

This was the stacktrace without my fix.

```
Traceback (most recent call last):
  File "/usr/local/bin/leeroy-cron", line 9, in <module>
    load_entry_point('leeroy==0.2.0', 'console_scripts', 'leeroy-cron')()
  File "/usr/local/lib/python2.6/dist-packages/leeroy/cron.py", line 59, in main
    retry_jenkins(repo_config, pull_request)
  File "/usr/local/lib/python2.6/dist-packages/leeroy/cron.py", line 34, in retry_jenkins
    log.debug("Creating a new Jenkins job for {}".format(pr_number))
ValueError: zero length field name in format
```

Ubuntu/Python 2.6