Retirement of Ruby 2.0.0 and switch default Ruby to 2.2

[Takahiro Kambe <taca%NetBSD.org@localhost>]

Hi,

Ruby 2.9.0 had become EOL 24th February 2016, so we should switch
default version of Ruby to 2.1 or later.

And I would change default version of Ruby to 2.2 since it has good
compatibility with 2.1 and looks stable.  (2.3 is a little bit early
to make it default version.)

Is there any comment?

-- 
Takahiro Kambe <taca%NetBSD.org@localhost>