Setting up elasticsearch with Haystack

I used to be using Whoosh as a search backend but now I'm switching to elasticsearch and trying to get things working.

When trying to rebuild the index I get the error:

requests.exceptions.ConnectionError: HTTPConnectionPool(host='localhost', port=8000): Max retries exceeded with url: /_bulk?op_type=create (Caused by <class 'socket.error'>: [Errno 61] Connection refused)

The following is in my settings.py:

HAYSTACK_CONNECTIONS = {
    'default': {
        'ENGINE': 'haystack.backends.elasticsearch_backend.ElasticsearchSearchEngine',
        'URL': 'http://localhost:8000/',
        'INDEX_NAME': 'haystack',
    },
}

My question is, what is URL used for and what do I put here? I'm running things locally for development and I'm deployed on Heroku.