how to setup Jekyll with Bourbon

Question

I'm trying to setup a Jekyll site with the Bourbon and Neat mixin libraries.

I created a _sass directory and installed Bourbon and Neat into this directory.

Then I import Bourbon and Neat in the main stylesheet (css/main.scss):

@import 'bourbon/bourbon';
@import 'neat/neat';

Then I'm trying out some mixins like span-columns() and shift(), but I'm getting this error:

Conversion error: Jekyll::Converters::Scss encountered an error while 
converting 'css/main.scss': Undefined mixin 'span-columns'. on line 2

Does anybody know why Jekyll does not recognize these Bourbon mixins?

my setup is at:

https://github.com/acandael/demo

thanks for your help!