Update math/gsl to 2.3

This release introduces some new features and fixes several bugs:
http://savannah.gnu.org/forum/forum.php?forum_id=8751

• update to 2.3 and take maintainership
• update math/py-gsl to 2.2.0 for gsl2 support
• update math/rubygem-rb-gsl to 2.1.0.2 for gsl2 support

PR: 218952
Exp-run by: antoine
Reviewed by: mat, rakuco
Approved by: rakuco (mentor)
Differential Revision: https://reviews.freebsd.org/D10522