Travis CI tests more Ruby versions.
Gemfile specifies "https://rubygems.org" for HTTPS.
Strongroom::Enigma loads YAML library, uses YAML.dump not #to_yaml.
Added "turn" gem for better test output.
has_rsa_key_spec uses Object.new not Minitest::Mock as sentinel.
Specs fixed for newer MiniTest: no #tap or #is_a? on Mocks.