TypeError: undefined is not a function (evaluating 'fs.absolute(require('system').args[3])')

I just did the following (running node v5.11.1):
```
backstop genConfig # seems to be successfull
```
And I'll get the followinging error:
```
$ backstop reference

BackstopJS CWD: /home/folder/www/project
BackstopJS loading config: /home/folder/www/project/backstop.json

COMMAND | Executing core for `reference`
clean | bitmaps_reference was cleaned.

Running CasperJS with: [ '/home/folder/npm/lib/node_modules/backstopjs/capture/genBitmaps.js' ]
CasperJS: I'm `fs` modules
CasperJS: TypeError: undefined is not a function (evaluating 'fs.absolute(require('system').args[3])')
CasperJS:
CasperJS: phantomjs://code/genBitmaps.js:5 in global code
CasperJS: :0 in injectJs
CasperJS: phantomjs://code/bootstrap.js:435
```

Reproduce the failure with Node v5.11.1 by running `backstop genConfig` followed by `backstop reference`. Start with `capture/genBitmaps.js`, especially the failing `fs.absolute(require('system').args[3])` call, and compare the available runtime APIs. Done means the reference command completes without this TypeError and generates the expected reference bitmaps.

Written by the indexing model from the issue text.