Remove WARN when using minitest5 and test unit

I am using rails 4.1, which includes minitest 5.
Getting this warning when running tests:
```ruby
MiniTest::Unit::TestCase is now Minitest::Test. From /Users/arthurnn/.rbenv/versions/2.1.1/lib/ruby/2.1.0/test/unit/testcase.rb:8:in `<module:Unit>'
```

This PR should fix the warning on test unit when using minitest 5+
review @zzak