Add broker-url and use in place of broker-host option

Thank you for writing such a useful recipe.  I really appreciate that I can have separate buildouts for multiple environments!

Newer versions of celery (testing with 3.0.13) provide a warning when BROKER_HOST is used in celeryconfig:

```
/home/atolber/projects/idigiload/eggs/celery-3.0.13-py2.7.egg/celery/utils/__init__.py:71: CPendingDeprecationWarning: 
    The 'BROKER_HOST' setting is scheduled for deprecation in     version 2.5 and removal in version v4.0.     BROKER_URL
```

Since this option is deprecated (it still works), I replace usage of broker-host from this recipe with broker-url.  broker-host will still inject BROKER_HOST (for backwards compatibility), it's just no longer documented.