drop dig_rb and update required ruby version to 2.3

[henrich]

ruby2.3 or higher version has dig_rb gem function as default, see top of https://github.com/jrochkind/dig_rb/blob/master/README.md

Now distros support ruby2.3 or higher version, bump up it would be okay, IMO.

• Debian9 or newer (Debian10 will be released 2 or 3 months later)
• Ubuntu16.04 or newer
• RHEL7 and its clone CentOS has ruby2.0, so out of the scope already ;)

[repeatedly]

Thanks for the patch.

The problem is we can't drop older ruby support without annoucement.
One idea is drop ruby 2.1/2.2 support in this year. In addition, drop one old ruby version in each year.
2020 drops ruby 2.3. 2012 drops ruby 2.4, ...

[henrich]

Make sense, I got it.
How about raise another issue to drop old ruby version and depends on it?

[ganmacs]

Now, we can drop dig-rb. could you update this branch?

[repeatedly]

#2750 includes these changes.
So we can close.

Thanks for the patch and suggestion!