Fix image asset urls

Hi,

I found the rake scripts are out of sync with the mapbox.css.scss file and the asset pipeline.

Firstly, the rake scripts convert .css to .sass and replace a few urls to adapt to Rails asset pipeline. I had to change the path of those asset pipeline commands.

Secondly, the rake script uses the .sass extension, but the file in the vendor folder was a .scss. Probably created without the script.

So, I fixed the script to use scss, the proper paths in image-url, and generated the mapbox.css.scss with the script.

I hope you can make use of it.

Martin