Switch default version of Ruby from 2.5 to 2.6.
For detailed list of changes, please check the Ruby website:
https://www.ruby-lang.org/en/news/2018/12/25/ruby-2-6-0-released/
How to upgrade is described in UPDATING entry 20190909.
Exp-Run: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=240094
Thanks to: antoine, sunpoet