LESS CSS strict maths update to be future proof

> We have introduced a optional strictMath mode, where math is required to be in parenthesis, e.g.

> (1 + 1)  // 2
> 1 + 1    // 1+1
> In 1.4.0 this option is turned off, but we intend to turn this on by default. We recommend you upgrade code and switch on the option (–strict-math=on in the command line or strictMath: true in JavaScript). Code written with brackets is backwards compatible with older versions of the less compiler.

> We also added a strict units option (strictUnits: true or strict-units=on) and this causes lessc to validate the units used are valid (e.g. 4px/2px = 2, not 2px and 4em/2px throws an error). There are no longer any plans to switch this option on permanently, but some users will find it useful for bug finding.

> Unit maths is done, so (4px * 3em) / 4px used to equal 3px and it now equals 3em. However we do not cancel units down to unitless numbers unless strict units is on. The selector interpolation, maths and units changes can be made to your less now and will compile fine with less 1.3.3.