delete unnecessary option of TestTask

The -r option for load "helper.rb" before tests are executed is works
well in 1.9.3 and 2.0.0, but there is some difference between 1.8.7 and
above. in 1.8.7, the "helper.rb" is loaded twice because it is required
in each test files already.

Rakefile

@@ -46,7 +46,6 @@ end
 require 'rake/testtask'
 Rake::TestTask.new do |t|
   t.libs << "test/" << "lib/"
-  t.ruby_opts << "-rhelper"
   t.test_files = FileList['test/test*.rb']
   t.verbose = true
 end