Address MRI 2.2 warnings (possible reference to past scope).

From https://github.com/ruby/ruby/commit/f5f6218a23fdbe45d1a80c202a131f02c3eef0ce.
It causes 280 warnings in our spec suite :(. The fix is to change the names of
variables and/or `let` definitions so they do not clash.

See https://bugs.ruby-lang.org/issues/10661 for more discussion.
Merge pull request #703 from rspec/address-2.2-issues

Address MRI 2.2 warnings (possible reference to past scope).