* Run tests for Ruby versions >= 2.6
* Remove not needed checks for methods defined
* Set required_ruby_version to >= 2.6
* Update READMEs with new Ruby versions
* Print warning on Rubies that require monkeypatching only
* Print warning to stderr instead of stdout
* Load ActiveSupport during internal testing only
* Add SINATRA_ACTIVESUPPORT_WARNING opt-out environment variable