TimeoutError when using "readyEvent": "backstopjs_ready"

On a newly initialized config the reference and test runs succeed. However, when I add "backstopjs_ready" to "readyEvent" I receive TimeoutError: timeout. If I revert by replacing "backstopjs_ready" with "", the runs succeed again.

```
macOS Sierra 10.12.6

Chrome: Version 61.0.3163.79 (Official Build) (64-bit)

$ node --version
v8.4.0

$ backstop --version
BackstopJS v3.0.24
BackstopJS 3.0.24

{
"id": "backstop_default",
"viewports": [
{
"label": "phone",
"width": 320,
"height": 480
},
{
"label": "tablet",
"width": 1024,
"height": 768
}
],
"onBeforeScript": "chromy/onBefore.js",
"onReadyScript": "chromy/onReady.js",
"scenarios": [
{
"label": "https://bpcoll.glynndevinsdigital.com/",
"cookiePath": "backstop_data/engine_scripts/cookies.json",
"url": "https://bpcoll.glynndevinsdigital.com/",
"referenceUrl": "",
"readyEvent": "backstopjs_ready",
"readySelector": "",
"delay": 0,
"hideSelectors": [
"#trackduck",
"#player"
],
"removeSelectors": [],
"hoverSelector": "",
"clickSelector": "",
"postInteractionWait": "",
"selectors": [],
"selectorExpansion": true,
"misMatchThreshold" : 0.1,
"requireSameDimensions": true
}
],
"paths": {
"bitmaps_reference": "backstop_data/bitmaps_reference",
"bitmaps_test": "backstop_data/bitmaps_test",
"engine_scripts": "backstop_data/engine_scripts",
"html_report": "backstop_data/html_report",
"ci_report": "backstop_data/ci_report"
},
"report": ["browser"],
"engine": "chrome",
"engineFlags": [],
"debug": false,
"debugWindow": false
}
```

Output
```
$ backstop reference
BackstopJS v3.0.24
Loading config: /Users/jroberts/Virtual/qa-render-test-app/sites/bpcoll.glynndevinsdigital.com/backstop.json

COMMAND | Executing core for `reference`
clean | backstop_data/bitmaps_reference was cleaned.
createBitmaps | Selected 1 of 1 scenarios.
Starting Chromy: port:9222 --disable-gpu,--force-device-scale-factor=1,--disable-infobars=true,--window-size=320,480
CREATING NEW REFERENCE FILES
Cookie state restored with: [
{
"path": "/",
"name": "yourCookieName",
"value": "yourCookieValue",
"expirationDate": 1798790400,
"hostOnly": false,
"httpOnly": false,
"secure": false,
"session": false,
"sameSite": "no_restriction",
"url": "https://.www.yourdomain.com"
}
]
SCENARIO > https://bpcoll.glynndevinsdigital.com/
Starting Chromy: port:9223 --disable-gpu,--force-device-scale-factor=1,--disable-infobars=true,--window-size=1024,768
CREATING NEW REFERENCE FILES
Cookie state restored with: [
{
"path": "/",
"name": "yourCookieName",
"value": "yourCookieValue",
"expirationDate": 1798790400,
"hostOnly": false,
"httpOnly": false,
"secure": false,
"session": false,
"sameSite": "no_restriction",
"url": "https://.www.yourdomain.com"
}
]
SCENARIO > https://bpcoll.glynndevinsdigital.com/
9222 LOG > JQMIGRATE: Migrate is installed with logging active, version 1.4.1
9222 LOG > console.trace
9223 LOG > JQMIGRATE: Migrate is installed with logging active, version 1.4.1
9223 LOG > console.trace
9222 LOG > backstopjs_ready
9223 LOG > backstopjs_ready
COMMAND | Command `reference` ended with an error after [33.776s]
COMMAND | BackstopException: https://bpcoll.glynndevinsdigital.com/ on undefined: TimeoutError: timeout
```

After I revert:

```
$ backstop reference
BackstopJS v3.0.24
Loading config: /Users/jroberts/Virtual/qa-render-test-app/sites/bpcoll.glynndevinsdigital.com/backstop.json

COMMAND | Executing core for `reference`
clean | backstop_data/bitmaps_reference was cleaned.
createBitmaps | Selected 1 of 1 scenarios.
Starting Chromy: port:9222 --disable-gpu,--force-device-scale-factor=1,--disable-infobars=true,--window-size=320,480
CREATING NEW REFERENCE FILES
Cookie state restored with: [
{
"path": "/",
"name": "yourCookieName",
"value": "yourCookieValue",
"expirationDate": 1798790400,
"hostOnly": false,
"httpOnly": false,
"secure": false,
"session": false,
"sameSite": "no_restriction",
"url": "https://.www.yourdomain.com"
}
]
SCENARIO > https://bpcoll.glynndevinsdigital.com/
Starting Chromy: port:9223 --disable-gpu,--force-device-scale-factor=1,--disable-infobars=true,--window-size=1024,768
CREATING NEW REFERENCE FILES
Cookie state restored with: [
{
"path": "/",
"name": "yourCookieName",
"value": "yourCookieValue",
"expirationDate": 1798790400,
"hostOnly": false,
"httpOnly": false,
"secure": false,
"session": false,
"sameSite": "no_restriction",
"url": "https://.www.yourdomain.com"
}
]
SCENARIO > https://bpcoll.glynndevinsdigital.com/
9222 LOG > JQMIGRATE: Migrate is installed with logging active, version 1.4.1
9222 LOG > console.trace
9223 LOG > JQMIGRATE: Migrate is installed with logging active, version 1.4.1
9223 LOG > console.trace
9222 LOG > backstopjs_ready
9223 LOG > backstopjs_ready

Run `$ backstop test` to generate diff report.

COMMAND | Command `reference` sucessfully executed in [6.965s]
```

Start with the readyEvent handling used by the scenario in backstop.json, then reproduce the issue with the shown BackstopJS 3.0.24 configuration by running backstop reference. Done means a configured backstopjs_ready event no longer causes TimeoutError and the reference command completes successfully.

Written by the indexing model from the issue text.