Less 1.4 Compat

Less 1.4 has a couple of breaking changes; namely, math needs to be wrapped in parens and mixed-unit math is disallowed. So I wrapped all the math in parens.

The one occurrence of mixed-unit math was in the calculation of `@gridsystem-width`. I removed the pixels unit but, honestly, I'm not sure why the value was being multiplied by 1px in the first place. I assumed it was to coerce less into rounding or something, but it didn't seem to be making a difference for me with 1.4 or 1.3.3. Which is just a roundabout way of saying that I'm not sure if b32266d is kosher, or whether something more complicated needs to be done (: Just in case, I kept it as a separate commit.