Support ed25519 SSH keys

[dentarg]

I'm using EdDSA keys at home. I'm thinking we can add the gems to the gemspec.

$ bundle exec cap production deploy
00:00 git:wrapper
      01 mkdir -p /tmp
(Backtrace restricted to imported tasks)
cap aborted!
NotImplementedError: unsupported key type `ssh-ed25519'
net-ssh requires the following gems for ed25519 support:
 * rbnacl (>= 3.2, < 5.0)
 * rbnacl-libsodium, if your system doesn't have libsodium installed.
 * bcrypt_pbkdf (>= 1.0, < 2.0)
See https://github.com/net-ssh/net-ssh/issues/478 for more information
Gem::LoadError : "rbnacl is not part of the bundle. Add it to your Gemfile."

Tasks: TOP => git:check => git:wrapper
(See full trace by running task with --trace)
The deploy has failed with an error: unsupported key type `ssh-ed25519'
net-ssh requires the following gems for ed25519 support:
 * rbnacl (>= 3.2, < 5.0)
 * rbnacl-libsodium, if your system doesn't have libsodium installed.
 * bcrypt_pbkdf (>= 1.0, < 2.0)
See https://github.com/net-ssh/net-ssh/issues/478 for more information
Gem::LoadError : "rbnacl is not part of the bundle. Add it to your Gemfile."
/Users/dentarg/.gem/ruby/2.4.2/gems/airbrussh-1.1.1/lib/airbrussh/console.rb:62: warning: constant ::Fixnum is deprecated

** DEPLOY FAILED

[dentarg]

Test:

ruby -rnet/ssh -e 'puts Net::SSH::Authentication::ED25519Loader::LOADED'