Fixes deprectated rdoc require.
Adds the MIT license to the gemspec.
Switches to minitest instead of shoulda.

* Bumps all development gems to eliminate deprecation warnings.
* The next commit will convert the tests to minispec syntax.
Mostly converts minitest to minispec syntax.

* Had some trouble with must_raise so punted on it for now.
* Replaces before block with a calls to `let`.
Adds support for Guard.