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.

The third commit in this pull request synchronizes the "kill test server" with the implementation in Resque. This allows Travis CI to properly kill the test server so that the tests pass there.