Consider dropping support for Ruby 2.0?

[ivoanjo]

Howdy!

I was doing some measurements around the installed size of debase-ruby_core_source and noticed that currently out of the installed size of 29M of the gem:

debase-ruby_core_source/lib/debase/ruby_core_source$ du -shc ruby-2.0.0-p*
396K	ruby-2.0.0-p0
396K	ruby-2.0.0-p195
396K	ruby-2.0.0-p247
396K	ruby-2.0.0-p353
396K	ruby-2.0.0-p451
400K	ruby-2.0.0-p481
400K	ruby-2.0.0-p576
400K	ruby-2.0.0-p594
400K	ruby-2.0.0-p598
400K	ruby-2.0.0-p643
400K	ruby-2.0.0-p645
400K	ruby-2.0.0-p647
400K	ruby-2.0.0-p648
5.1M	total

... 5.1M (or around 17%) is Ruby 2.0 sources.

Given that there's not going to be any more 2.0 releases, what do you think of:

1. Removing the 2.0 sources
2. Bumping the required ruby version to 2.1

This way, Ruby 2.0 users could still install the gem (they would get the latest version that still had 2.0), but newer versions could get a few space savings.

(This could even be expanded to include every Ruby for which there will never be any new release, but I decided to start with 2.0 since that one is really really old and there's a lot of releases being included)

If this sounds good, I'd be happy to put in the PRs and help in any way to get this out the door.