add "sourceRoot"

``` js
cssmin: {
    options: {
        sourceMap: true,
        srcRoot: '../../' //add
    },
    target: {
        files: {
            'build/css/main.min.css': 'src/css/*.css'
        }
    }
},
```

`sourceRoot`: Optional. The URL root from which all sources are relative.
https://github.com/mozilla/source-map/

Thanks
