less: Move to new selector interpolation, supported from 1.3.1

less introduced a better selector interpolation format in 1,3,1. It will remove the old (undocumented) format in 1.4.0.

This pull request updates you to be compatible with future versions of less.

I am one of the core mantainers of less.

(had this on master then read the guidelines and moved to 2.3.0-wip - is that right?)

p.s. I can also remove those ugly 

    .spanX (0) {}

they are not needed in 1.3.3 but it depends what version of less you want to support.