* adding yield at end of run block, to allow sensu-server shutdown

Without adding this yield, I could never shutdown sensu-server using ```/etc/init.d/sensu-server restart```

In the logs, you'd see this, where *handlers_in_progress_count* would be a higher value, depending on how many influxdb handler calls were made.   

This patch allows me to restart OK.

```
{"timestamp":"2014-12-26T17:41:42.569722+0000","level":"warn","message":"received signal","signal":"TERM"}
{"timestamp":"2014-12-26T17:41:42.570002+0000","level":"warn","message":"stopping"}
{"timestamp":"2014-12-26T17:41:42.570101+0000","level":"warn","message":"unsubscribing from keepalive and result queues"}
{"timestamp":"2014-12-26T17:41:42.570391+0000","level":"warn","message":"resigning as master"}
{"timestamp":"2014-12-26T17:41:42.570501+0000","level":"info","message":"completing handlers in progress","handlers_in_progress_count":2}
```