Slack bot calls deprecated RTM.Start method

RTM.Start was deprecated, calling it throws a "method_deprecated" error. Replacing the `rtm.start` API call with `rtm.connect` fixes it.

In `lib/Slack_web_api.js`:
```
-          slack_api.callAPI('rtm.start',options,cb);
+          slack_api.callAPI('rtm.connect',options,cb);
```