Profile option not working

I have a Firefox profile inside my project directory, which can be used using `firefox -profile ffprofile` but when I add into backstop.js it's not working

```
"casperFlags": [
"--profile ffprofile",
],
"engine": "slimerjs",
```

Tried `"-profile ffprofile"` as well, if I execute casperjs manually it does work `./node_modules/.bin/casperjs --engine=slimerjs --profile ./ffprofile`