new port: math/rubygem-expression_parser

A mathematical expression parser for Ruby

WWW: http://lukaszwrobel.pl/blog/math-parser-part-1-introduction

PR: 199923
Differential Revision: https://reviews.freebsd.org/D2483
Submitted by: Torsten Zuehlsdorff <ports@toco-domains.de>
Approved by: mat (mentor)