generated main.css is empty when using compass and building with grunt build

`grunt build` generates an empty `dist/styles/*.main.css` file. 

If I execute the grunt tasks one by one everything would be fine. However calling `grunt build` shows me an output like this:

    Running "cssmin:dist" (cssmin) task
    File dist/styles/main.css created.
    
    Running "cssmin:dist/styles/main.css" (cssmin) task
    File dist/styles/main.css created.
    
or this
    
    Running "concat:dist" (concat) task
    File "dist/scripts/scripts.js" created.
    
    Running "concat:dist/styles/main.css" (concat) task
    File "dist/styles/main.css" created.
    
    Running "concat:dist/scripts/scripts.js" (concat) task
    File "dist/scripts/scripts.js" created.

Have you noticed the `"concat:dist/styles/main.css"` - that's wrong and results in overwritting the `main.css` file.

I don't understand why this happens, but I've fixed this locally by exchanging the references to the `cssmin` and `concat` task in the `build` task registration by `cssmin:dist` and `concat:dist`.