Document "doc" task and use "sh" so it prints output

Rakefile

@@ -7,11 +7,11 @@ Cucumber::Rake::Task.new(:cucumber, 'Run features that should pass') do |t|
   t.cucumber_opts = "--color --tags ~@wip --strict --format #{ENV['CUCUMBER_FORMAT'] || 'pretty'}"
 end
 
-require 'rake/testtask'
 require 'rake/clean'
 
 task :test => ["cucumber"]
 
+desc "Build HTML documentation"
 task :doc do
-  `bundle exec yard`
-end
+  sh 'bundle exec yard'
+end