Skip adding lib directory to libs explicitly

It seems `lib` directory is included in `test.libs` implicitly.

Rakefile

@@ -4,7 +4,7 @@ Bundler::GemHelper.install_tasks
 require 'rake/testtask'
 
 Rake::TestTask.new(:test) do |test|
-  test.libs << 'lib' << 'test'
+  test.libs << 'test'
   test.test_files = FileList['test/plugin/*.rb']
   test.verbose = true
 end