Fix crash which occurs when pinging. (#1707)
Fix crash which occurs when pinging. (#1707)

```
2019-03-15 09:56:24.233 ERROR  : /Users/mstegeman/workspace/gateway/build/webpack:/src/ssltunnel.js:190
      `/ping?token=${this.tunneltoken.token}`;
                     ^
2019-03-15 09:56:24.234 ERROR  : TypeError: Cannot read property 'token' of undefined
    at Object.pingRegistrationServer (/Users/mstegeman/workspace/gateway/build/webpack:/src/ssltunnel.js:190:22)
    at Timeout.pingInterval.setInterval [as _onTimeout] (/Users/mstegeman/workspace/gateway/build/webpack:/src/ssltunnel.js:123:1)
    at ontimeout (timers.js:498:11)
    at tryOnTimeout (timers.js:323:5)
    at Timer.listOnTimeout (timers.js:290:5)
```
mozilla-iot/gateway,mozilla-iot/gateway,moziot/gateway,moziot/gateway,mozilla-iot/gateway,mozilla-iot/gateway,moziot/gateway,mozilla-iot/gateway