gemspec_compatible in smtp_mock-1.3.0

- old
+ new

@@ -21,16 +21,17 @@
     'documentation_uri' => 'https://github.com/mocktools/ruby-smtp-mock/blob/master/README.md',
     'bug_tracker_uri' => 'https://github.com/mocktools/ruby-smtp-mock/issues'
   }
 
   spec.required_ruby_version = '>= 2.5.0'
+  dry_struct_version = ::Gem::Version.new(::RUBY_VERSION) >= ::Gem::Version.new('2.7.0') ? '~> 1.6' : '~> 1.4'
 
   spec.files         = `git ls-files -z`.split("\x0").reject { |f| f.match(%r{^(test|spec|features)/}) }
   spec.executables   = %w[smtp_mock]
   spec.require_paths = %w[lib]
 
-  spec.add_runtime_dependency 'dry-struct', '~> 1.4'
+  spec.add_runtime_dependency 'dry-struct', dry_struct_version
 
   spec.add_development_dependency 'ffaker', '~> 2.21'
   spec.add_development_dependency 'rake', '~> 13.0', '>= 13.0.6'
-  spec.add_development_dependency 'rspec', '~> 3.11'
+  spec.add_development_dependency 'rspec', '~> 3.12'
 end