Replace Test::Unit With Minitest

- Add minitest dependency to Gemfile test group
- Refactor `TestHelpers` and mixin to `Minitest::Test`
  - Remove unused `assert_doesnt_include`
  - Remove `assert_include` since `Minitest` defines `assert_includes`
- Replace `Test::Unit` methods with matching `Minitest` methods
- Refactor `assert_nothing_raised` to use `assert_equal` since
  `Minitest` doesn't define `assert_nothing_raised`
  - See http://blog.zenspider.com/blog/2012/01/assert_nothing_tested.html
- Add Travis CI build matrix
  - Allow REE to fail since charlock_holmes isn't installing
  - Fast finish so build doesn't wait for REE to fail
Manually merge pull request #126 from minitest

Replace Test::Unit With Minitest
- Squashed exploratory commits