Revert "Rack 2.x requirements Ruby 2.2.2+"
Revert "Rack 2.x requirements Ruby 2.2.2+"

This reverts commit 6aae6377c75b9f4ee2eee7264e70aef6ef25ccb1.

Refer: https://github.com/haml/html2haml/pull/73#issuecomment-270109298,
http://sue445.hatenablog.com/entry/2016/09/02/121047

koic/active_pstore