Wrap spec method in DSL module.

MiniTest 4.7.x introduced the DSL module and minitest-spec-rails (among others) use it. I can't use this extension with minitest-spec-rails without this change. All tests pass and the README on this project already says that it requires 4.7.x so I think we're good to go. See metaskills/minitest-spec-rails#32 for more details.