Update ReportTrafficWorker to stop using deprecated API

System reports the System's API traffic to Backend using the Ruby client for the Service Management API, and it hits the following deprecation messages which fill up the logs:

provider keys are deprecated - please switch at your earliest convenience to use service tokens

warning: def report(*transactions) is deprecated. In next versions, the signature of the report method is going to be: def report(transactions: [], service_id: nil)

See https://github.com/3scale-labs/3scale_ws_api_for_ruby/blob/master/lib/3scale/client.rb#L48-L54

Indeed, the client is initialized with the `provider_key`: https://github.com/3scale/porta/blob/8f9124b/app/workers/report_traffic_worker.rb#L91-L92

The ReportTrafficWorker needs to be adjusted to use the non-deprecated interface to avoid so the deprecation logs.