tests: switch to minitest 5

The first commit in this pull request switches resque-cleaner's test suite to Minitest. (Ruby 1.9+ uses Minitest as the backend for Test::Unit. As of Minitest 5, the shim has broken some compatibility with Test::Unit::TestCase in some scenarios.)

The second commit in this pull request removes the dependency on resque/server/test_helper. The Resque developers have removed `resque/server/test_helper.rb` in Resque's master branch when they split out resque-web (https://github.com/resque/resque/commit/b2ca4ceb9457242fc0170d95b3bce9d427baa387). It seems that this test_helper is an internal implementation detail and that external consumers should not rely on it. Additionally, in Resque v1.25.2, `resque/server/test_helper.rb` adjusts the Test::Unit::TestCase class. Now that resque-cleaner uses Minitest, we want to adjust the Minitest::Spec class instead.