Update app_metrics/backends/redis.py

I'm trying out app_metrics backed by celery and redis, and I'm getting this exception in celery:

    Task app_metrics.tasks.redis_metric_task[7355cd97-bda0-4e88-b4e7-c1cb960b67c8] raised exception: TypeError('redis_metric_task() takes at most 2 arguments (3 given)',)

Should `properties` be a mapping here instead of `None`? Does the redis backend even support additional properties for metrics? I'm still trying to wrap my head around all this, so please forgive me if I'm asking silly questions ;)
Merge pull request #27 from bradmontgomery/patch-1

Update app_metrics/backends/redis.py