Move rake task definition into #define.
Moved ivar initialization back into #initialize.
Merge pull request #1493 from postmodern/code_smell

Code smell
Restrict minitest to 5.x.

We're not sure yet what minitest 6.x will entail. So we're locking the
gem to a 5.x version.
Include feature spec on desired behavior.

Mark feature as `@wip` so it doesn't fail the build while the feature is
being implemented.
Replace `stdlib` with `minitest` & `test_unit`.

- Remove support for the `stdlib` expectation framework
- Include `test_unit`
- Include `minitest`
- In specs treat previous `stdlib` as `minitest`
- Attempt to require the 'minitest' gem
  - Supports minitest with Ruby 1.8.7
  - Supports minitest 5.x which is not part of stdlib
Remove wip tag.
Note behavior change in changelog.