Allow rbnacl 4 and better description on what to install

[mfazekas]

Closes: #476

Improves error message if required dependencies not yet installed.

$ BUNDLE_GEMFILE=./Gemfile.norbnacl bundle exec irb
2.3.0 :001 > require 'net/ssh'
 => true 
2.3.0 :002 > Net::SSH.start('localhost','foo', {keys: 'test/keys/id_ed25519_pwd'})
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 Gemfile."