Update variables.less

This change forces the less javascript program (I am using less-1.4.1.min.js) to evaluate the espression in a media selector like : @media (max-width: @navbarCollapseDesktopWidth).
Whithout the parenthesizes, the output of the above command is : @media (max-width: 979px + 1) instead of @media (max-width: 980px).

Best regards