switched to using maps in sass for paths

Implements the suggestion here https://github.com/Financial-Times/o-assets/issues/2

I also removed explicit need to specify versions. I don't think we need it as all it's used for is to build a path... so why not just anticipate that the version might be included in the path? Am I missing something here?

@triblondon @dan-searle @AlbertoElias @samgiles 

@pornel - would this implementation work with your libsass hack?