Log message:
lang/ruby26-base: add Ruby 2.6.1

Add Ruby release 2.6.1.

Ruby 2.6.0 (2018-12-25)

New major features:

* A new JIT compiler.
* The RubyVM::AbstractSyntaxTree module.

For more detail, please refer: \ 
<https://www.ruby-lang.org/en/news/2018/12/25/ruby-2-6-0-released/>

Ruby 2.6.1 (2019-01-30)

Major change:

* Net::Protocol::BufferedIO#write raises NoMethodError when sending large
  multi-byte string has been fixed.

For more detail, please refer: \ 
<https://www.ruby-lang.org/en/news/2019/01/30/ruby-2-6-1-released/>