Fixed minitest-rails capability

`minitest/rails/testing` doesn’t exist anymore and there is a shorter way to declare a new test task
More docs: https://github.com/blowmage/minitest-rails/blob/cf2302bcec7d7620819c744e84e26c4452eed4be/README.rdoc#running-tests

Otherwise it's raising with 
```
LoadError: cannot load such file -- minitest/rails/testing
``