Resolve gem build warnings

[synth]

gem build gives the following output we should resolve at some point

$ gem build
WARNING:  open-ended dependency on debug (>= 1.0.0, development) is not recommended
  if debug is semantically versioned, use:
    add_development_dependency "debug", "~> 1.0", ">= 1.0.0"
WARNING:  make sure you specify the oldest ruby version constraint (like ">= 3.0") that you want your gem to support by setting the `required_ruby_version` gemspec attribute
WARNING:  You have specified the uri:
  https://github.com/synth/rails_mail
for all of the following keys:
  homepage_uri
  source_code_uri
Only the first one will be shown on rubygems.org
WARNING:  See https://guides.rubygems.org/specification-reference/ for help
  Successfully built RubyGem
  Name: rails_mail
  Version: 0.12.0
  File: rails_mail-0.12.0.gem