[Bug][Chrome][Randomly] Error: Unknow event id 14 & 17

Hello All,

We found the following error appearing randomly during our test ( we have 300 links need to be tested).
And we checked the captured page, some captured pages cannot totally captured.
Appear: 4 times - 3 "Unknow event id 17", and 1 "unknow event id 14"

```
Error: Unknown event id: 17
at /usr/local/lib/node_modules/backstopjs/node_modules/chrome-remote-interface/lib/chrome.js:81:28
at Chrome.handleMessage (/usr/local/lib/node_modules/backstopjs/node_modules/chrome-remote-interface/lib/chrome.js:271:13)
at WebSocket. (/usr/local/lib/node_modules/backstopjs/node_modules/chrome-remote-interface/lib/chrome.js:248:27)
at emitTwo (events.js:126:13)
at WebSocket.emit (events.js:214:7)
at Receiver._receiver.onmessage (/usr/local/lib/node_modules/backstopjs/node_modules/ws/lib/WebSocket.js:143:54)
at Receiver.dataMessage (/usr/local/lib/node_modules/backstopjs/node_modules/ws/lib/Receiver.js:385:14)
at extension.decompress (/usr/local/lib/node_modules/backstopjs/node_modules/ws/lib/Receiver.js:354:40)
at _inflate.flush (/usr/local/lib/node_modules/backstopjs/node_modules/ws/lib/PerMessageDeflate.js:279:12)
at afterWrite (_stream_writable.js:454:3)
```

```
Error: Unknown event id: 14
at /usr/local/lib/node_modules/backstopjs/node_modules/chrome-remote-interface/lib/chrome.js:81:28
at Chrome.handleMessage (/usr/local/lib/node_modules/backstopjs/node_modules/chrome-remote-interface/lib/chrome.js:271:13)
at WebSocket. (/usr/local/lib/node_modules/backstopjs/node_modules/chrome-remote-interface/lib/chrome.js:248:27)
at emitTwo (events.js:126:13)
at WebSocket.emit (events.js:214:7)
at Receiver._receiver.onmessage (/usr/local/lib/node_modules/backstopjs/node_modules/ws/lib/WebSocket.js:143:54)
at Receiver.dataMessage (/usr/local/lib/node_modules/backstopjs/node_modules/ws/lib/Receiver.js:385:14)
at extension.decompress (/usr/local/lib/node_modules/backstopjs/node_modules/ws/lib/Receiver.js:354:40)
at _inflate.flush (/usr/local/lib/node_modules/backstopjs/node_modules/ws/lib/PerMessageDeflate.js:279:12)
at afterWrite (_stream_writable.js:454:3)
```

**System Info**
1. Backstop: 3.0.38
2. Node: 8.9.4
3. NPM: 5.6.0
4. OS: Centos

Thank you,
Ken