Goodbye Travis CI

[y-yu]

• I cannot fix that all CIs except for Emacs 26.3 were down 😇
  • 
• I think it caused due to something cryptographic errors like SSL/TLS 🤔
  • This Stack Exchange comment says that we have to put the latest package-keyring.gpg into etc directory in older Emacs than 26.3
  • But the errors are still being occurred
• In nix-emacs-ci, download the latest package-keyring.gpg 👀
  • https://github.com/purcell/nix-emacs-ci/blob/be82e2f2df127ec6204885019f66984ef28745fd/emacs.nix#L71-L72