Can't make it work via config.rb

Getting that dreaded `Individual stylesheets must be in sass dir` error. I can make it work without `config.rb`, but for obvious reasons, prefer to use the configuration file.

`config.rb`:
```
project_path = 'demo'
sass_dir = 'sass'
css_dir = 'css'
```

File-system:
```
./demo/sass
./demo/css
```

Gulp-compass config:
```
compass({
config_file: 'config.rb'
}
```

I don't pass Sass/Css since the defaults are what I use anyway.

Reproduce the error using the shown config.rb, demo/sass and demo/css directories, and the gulp-compass config_file setting. Start by tracing how config_file is resolved and how project_path, sass_dir, and css_dir are interpreted; done means the configuration works without the “Individual stylesheets must be in sass dir” error.

Written by the indexing model from the issue text.