Fix #102 once #stub is removed

This should be a comment on #102 but @github keeps telling me that my comment is blank, which it obviously isn't. Sorry about that.

`#stub` and `#mock` will be removed in v3, so we can then include ExampleMethods (together with ArgumentMatchers) into the ExampleGroup class instead of the instance's meta class. @justinko found out that this would fix #102.

I've done the necessary changes in this branch (including removing `#stub` and `#mock`): https://github.com/michihuber/rspec-mocks/compare/make-let-override-argument-matchers

I can send a pull request once the 3.0 branch is open. /cc @myronmarston