* I did not know about ::Rails::VERSION::STRING until recently. It seems
  to be defined all the way back to 3.0.0, which is all we need.

…rative method

There is currently some odd interactions between rspec-rails and
rspec-core when it comes to `let` definitions.

If we call `subject` in an `included` hook, the dynamically generated
`LetDefinitions` module gets inserted into the ancestor chain earlier
than normal. This placement no longer allows `let` definitions to
override methods defined on other modules that are brought in via
`config.include`.

We may raise a warning in future (see discussion in #738)

[Fixes #738]

Conflicts:
	Changelog.md

* Has since been fixed in rails/rails#10584

[Fixes #745]

[ci skip]