Use Brick/Math

[BallisticPain]

Brick/Math keeps the focus of php-units-of-measure out of the math bit... It will use the fastest available math module installed whether that's BCMath or GMP or neither.

I'm looking to supply a PR for this. (working on it)

This would require a major version number change ... PHP 7.1 or newer is required
Version 0.5 handles older versions of PHP

Thanks for a great library!