delete unnecessary option of TestTask

The -r option for load "helper.rb" before tests are executed is workswell in 1.9.3 and 2.0.0. 
But the operation of "require" has 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.
I think it is the problem that the travisCI is failed.