Require its specific version in `Gemfile`.
Fix it's installation in CI.
Enable new cops.
Resolve new offenses.
Drop support of Ruby 2.3
(it's not supported by RuboCop and by MRI:https://www.ruby-lang.org/en/news/2019/03/31/support-of-ruby-2-3-has-ended/)
Deprecate `Faraday::Request#method`, replace it with `#http_method`.
Spins up a proxy server in a separate process, runs a few requests
through it with each adapter to verify that both proxy support and basic
authentication take effect.
