Fix TypeError when enable timezone

Hubot occurred following error when enable timezone.

```
2014-12-09T06:15:00.203359+00:00 app[web.1]: [Tue Dec 09 2014 06:15:00 GMT+0000 (UTC)] ERROR TypeError: Cannot call method 'send' of undefined
2014-12-09T06:15:00.203365+00:00 app[web.1]:   at Job.sendMessage (/app/node_modules/hubot-cron/src/scripts/cron.coffee:127:5, <js>:177:20)
2014-12-09T06:15:00.203367+00:00 app[web.1]:   at Object.<anonymous> (/app/node_modules/hubot-cron/src/scripts/cron.coffee:115:7, <js>:158:22)
2014-12-09T06:15:00.203369+00:00 app[web.1]:   at Object.CronJob._callback (/app/node_modules/hubot-cron/node_modules/cron/lib/cron.js:339:26)
2014-12-09T06:15:00.203371+00:00 app[web.1]:   at [object Object].callbackWrapper [as _onTimeout] (/app/node_modules/hubot-cron/node_modules/cron/lib/cron.js:401:14)
2014-12-09T06:15:00.203373+00:00 app[web.1]:   at Timer.listOnTimeout [as ontimeout] (timers.js:112:15)
```

So, we added `robot` to [the line](https://github.com/miyagawa/hubot-cron/blob/master/src/scripts/cron.coffee#L55).