Spec failure due to Bundler not allowing `async-dns` to require `base64`

[postmodern]

Bundler no longer allows requiring base64, unless it is added as a gem dependency. This breaks async-dns which requires base64.

LoadError:
  cannot load such file -- base64
# ./vendor/bundle/ruby/3.4.0/gems/async-dns-1.3.0/lib/async/dns/message.rb:24:in '<top (required)>'
# ./vendor/bundle/ruby/3.4.0/gems/async-dns-1.3.0/lib/async/dns.rb:27:in 'Kernel#require_relative'
# ./vendor/bundle/ruby/3.4.0/gems/async-dns-1.3.0/lib/async/dns.rb:27:in '<top (required)>'
# ./lib/ronin/dns/proxy/server.rb:24:in '<top (required)>'

https://github.com/ronin-rb/ronin-dns-proxy/actions/runs/13103082741/job/36553753530

See upstream issue: socketry/async-dns#35

[postmodern]

Waiting on socketry/async-dns#36 to be merged.

[postmodern]

async-dns 1.4.0 has been released which drops the dependency on the base64 library.