gem: use bundler-audit

Rakefile

@@ -1,6 +1,8 @@
 require 'bundler/gem_tasks'
 require 'rspec/core/rake_task'
+require 'bundler/audit/task'
 
 RSpec::Core::RakeTask.new(:spec)
+Bundler::Audit::Task.new
 
-task :default => :spec
+task :default => [:spec, 'bundle:audit']