LetDefinitions needs to be defined on the nested instance rather than the definition of ExampleGroup

Somehow `rspec-rails` is changing the order of module definition, such that the `let` defintion brought in by @samphippen to give us awesome nicities like `super` (see [here](https://github.com/rspec/rspec-core/blob/dc8166077b465da17a8ddcc26cd37e4f8f79f937/lib/rspec/core/memoized_helpers.rb#L479)) is broken when used with `rspec-rails`.

The symptom of this originally popped up in `rspec-core` (rspec/rspec-core#817) but I was unable to replicate it without `rspec-rails` and I'm pretty certain it's an issue with `rspec-rails`.

I've created this branch on `rspec-rails` rather than my fork because I need feedback on how this could be occurring, I'm not familiar enough with the internals of `rspec-rails`. The feature attached is not expected to be included in any final patch, it's just for demonstation of the problem.