Bugfix for media query width calculation

LESS 1.4 needs strictMath.

(1 + 1)  // 2
1 + 1    // 1+1
LESS 1.4 fix

strictMath calculation fix (same as previous commit)
Merge pull request #6 from lekkerduidelijk/patch-1

Bugfix for media query width calculation: strictMath compliance.
Merge pull request #7 from lekkerduidelijk/patch-2

LESS 1.4 fix: strictMath compliance.
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.