Log Message:
lang/ruby: change default Ruby to 2.6.x

Change default vesion of Ruby from 2.4.x to 2.6.x.

* Ruby 2.7 will be released within this year.
* Ruby 2.6.x is stable enough and actively maintained.
* Ryby 2.5.x will be in security maintenance phase after
  release of Ruby 2.7.
* Ruby 2.4.x will be EOL after 31th March 2020.