Make file is outdated

[StevenTso]

The parameter --yui-compress for lessc is now deprecated. It has been replaced with the parameter --clean-css.

For reference:
https://github.com/less/less.js/blob/9d3ef59cf818691cd52410879e7152e7d589de32/lib/less-rhino/index.js#L237

To fix this issue:
We could change the Makefile to take the parameter --clean-css or no parameter at all. If we were to go with the parameter of --clean-css, we would have an additional dependency of having to install
'npm install -g less-plugin-clean-css'