Completes but does not produce anything

How do I debug BackstopJS, to find why it does not produce anything?

Console output in `debug` mode:

```
[19:11:16] Starting 'clean'...
bitmaps_reference was cleaned.
[19:11:16] Starting 'bless'...
[19:11:16] Finished 'bless' after 11 ms
[19:11:16] Finished 'clean' after 33 ms
[19:11:16] Starting 'reference'...
gulp.run() has been deprecated. Use task dependencies or gulp.watch task triggering instead.
[19:11:16] Starting 'init'...
[19:11:16] Finished 'init' after 7.25 μs
[19:11:16] Starting 'test'...

Generating reference files.

Running CasperJS with: [ 'capture/genBitmaps.js' ]
[19:11:16] Finished 'test' after 18 ms
reference has run.
[19:11:16] Finished 'reference' after 19 ms
CasperJS: Debug is enabled!
CasperJS: CREATING NEW REFERENCE FILES
CasperJS: onBeforeEach.js
CasperJS: onBeforeEach.js
CasperJS: onBeforeEach.js
CasperJS: Remote Error > TypeError: Type error
CasperJS: file: https://static.twilio.com/libs/twiliojs/refs/5c1f1e8/twilio.min.js
CasperJS: line: 236
function: error

Bitmap file generation completed.

Run `$ npm run test` to generate diff report.
```