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.
Merge pull request #108 from takashi/delete-unnecessary-option

delete unnecessary option of TestTask