Drop Ruby 2.1 support
Drop Ruby 2.1 support

Ruby 2.1 is EOL as of March; for details, see
https://www.ruby-lang.org/en/news/2016/02/24/support-plan-of-ruby-2-0-0-and-2-1/

barraq/nanoc,RubenVerborgh/nanoc,nanoc/nanoc