Fix @line-height-ratio calculation bug caused by strictMath mode in LESS 1.4

This is a fix for another issue that has cropped up due to the strictMath mode
introduced in LESS 1.4.

The @line-height-ratio variable evaluates to 24/16 instead of 1.5. This commit
adds parentheses around the division operation to fix the issue.
Merge pull request #8 from drakeh/LESS

Fix @line-height-ratio calculation bug caused by LESS 1.4 strictMath