Update CSS compiler to match latest ElmCSS docs

[bdugan14]

I get the following errors when I run elm-css Stylesheets.elm before making any changes in tutorial part13:

-- NAMING ERROR ------------------------------------------------ Stylesheets.elm

Cannot find type `Model`

16| main : Program Never Model Msg
                         ^^^^^

-- NAMING ERROR ------------------------------------------------ Stylesheets.elm

Cannot find type `Msg`

16| main : Program Never Model Msg
                               ^^^

Detected errors in 1 module.                                        
Error: Errored with exit code 1

If I update Stylesheets.elm to match the ElmCss docs (special module in "Approach 2"), it works fine.