Backstop raising error on COMMAND | Executing core for `report`

Is that possible that some dependency is missing in OS? Debian env on Heroku.

```
COMMAND | Executing core for `report`
events.js:183
throw er; // Unhandled 'error' event
^
Error: Invalid file signature
at module.exports.Parser._parseSignature (/app/node_modules/node-resemble-js/node_modules/pngjs/lib/parser.js:51:18)
at module.exports.ChunkStream._processRead (/app/node_modules/node-resemble-js/node_modules/pngjs/lib/chunkstream.js:174:13)
at module.exports.ChunkStream._process (/app/node_modules/node-resemble-js/node_modules/pngjs/lib/chunkstream.js:193:14)
at module.exports.ChunkStream.write (/app/node_modules/node-resemble-js/node_modules/pngjs/lib/chunkstream.js:61:8)
at exports.PNG.PNG.write (/app/node_modules/node-resemble-js/node_modules/pngjs/lib/png.js:92:16)
at ReadStream.ondata (_stream_readable.js:639:20)
at emitOne (events.js:116:13)
at ReadStream.emit (events.js:211:7)
at addChunk (_stream_readable.js:263:12)
at readableAddChunk (_stream_readable.js:250:11)
```

**System:**
NODE: 8.9.2
Platform: Ubuntu 16.04 (on Heroku)
Backstop: v3.0.38

Start by reproducing the `report` command on the stated Heroku Debian environment and inspect the PNG parsing stack under `node_modules/node-resemble-js/node_modules/pngjs/`, especially `parser.js` and `png.js`. Compare the input being read during report execution with the expected PNG signature; done means the report completes without the `Invalid file signature` error or the dependency/environment cause is identified.

Written by the indexing model from the issue text.