gem: add bundler-audit for vulnerability scan

Rakefile

@@ -1,8 +1,10 @@
 require 'bundler/gem_tasks'
 require 'rspec/core/rake_task'
 require 'rubocop/rake_task'
+require 'bundler/audit/task'
 
 RSpec::Core::RakeTask.new(:spec)
 RuboCop::RakeTask.new
+Bundler::Audit::Task.new
 
-task default: [:rubocop, :spec]
+task default: [:rubocop, :spec, 'bundle:audit']

dyndnsd.gemspec

@@ -32,4 +32,5 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'rspec'
   s.add_development_dependency 'rack-test'
   s.add_development_dependency 'rubocop', '~> 0.60.0'
+  s.add_development_dependency 'bundler-audit'
 end