Update ruby-rbnacl to 7.1.1.
## [7.1.1] (2020-01-27)
- Test on Ruby 2.7 ([#208])
- Add project metadata to the gemspec ([#207])
- Resolve FFI deprecation warning ([#206])
## [7.1.0] (2019-09-07)
- Attached signature API ([#197], [#202])
- Fix the `generichash` state definition ([#200])
## [7.0.0] (2019-05-23)
- Drop support for Ruby 2.2 ([#194])
## [6.0.1] (2019-01-27)
- Add fallback `sodium_constants` for Argon2 ([#189])
- Support libsodium versions used by Heroku ([#186])
- Sealed boxes ([#184])
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