Add minitest 4.7.x as a dev dependency.

Using 4.7.x instead of 5.x to keep the cucumber version happy.
Include feature spec on stdlib deprecation.

These features are the only ways to ensure the proper integration of
minitest and test/unit compatibility between versions. The specs do not
load the libs as that would pollute the other specs.
Deprecate `stdlib` for `minitest` & `test_unit`.

- Deprecate support for the `stdlib` expectation framework
- Include `test_unit`
- Include `minitest`
- In specs treat previous `stdlib` as `minitest`; except those
  confirming backwards compatibility
- Add support for the minitest + test/unit combos.

  There are a lot of subtle issues with the different combinations of
  versions of Ruby + test/unit + minitest. These can cause load and name
  errors.

  The newer Ruby versions (1.9+) use test/unit as a light shim around
  minitest. Due to this, they load `MiniTest`, note the upcase on
  'Test', when test/unit is loaded.

  If either Ruby 1.8 is used or the 'test_unit' gem is loaded,
  'minitest' is not be loaded.

  If the Minitest 5.x gem is loaded, it shims over `MiniTest` using the
  new constant `Minitest` (currently for backwards compatibility with
  Ruby core it aliases itself with `MiniTest`).

  The only situation where support for Minitest 5.x is required, is if
  the Minitest 5.x gem is loaded.

RSpec 3.x does not provide explicit support for Minitest 4.x. However,
due to differences in `require` paths, we provide a fallback option in
the case it is used. We are not in the business of dictating which
versions our users should or should not use, nor on how they setup their
load paths (i.e. bundler or other methods).
Note `stdlib` deprecation in Changelog.
Merge pull request #1501 from rspec/split-expect_with-stdlib-2-99

Deprecate `expect_with :stdlib` in favour of `expect_with :test_unit` and `expect_with :minitest`