Remove default value duplication in rakefile. (rails#1559)

Author: josh08h

Rakefile

@@ -2,9 +2,8 @@
 require "bundler/gem_tasks"
 require "rake/testtask"
 
-Rake::TestTask.new(:test) do |t|
+Rake::TestTask.new do |t|
   t.libs << "test"
-  t.libs << "lib"
   t.test_files = FileList["test/**/*_test.rb"]
   t.verbose = true
 end