Make compatible with LESS 1.4

Created by: Piedone

LESS 1.4 is currently beta but it wouldn't harm to prepare for its breaking changes. I only found problematic mathematical expressions that won't compile (correctly) with 1.4.0b (using the latest BS source):

• panels ln 20 should be enclosed in parentheses
• .make-column() will produce a width with NaN% for @media (min-width: 768px)