Deal with 1.8.7 any_instance block deprecation warnings.

The metadata provided by 1.8's `Proc` objects do not
provide sufficient information to only issue this
deprecation warning when it's truly necessary.
See cd3208a79fed0f0ea9bf22f34ef8b6ebd04fec2b for
the background about why.

The best we can do is work around these warnings.
Merge pull request #555 from rspec/fix-issue-554

Deal with 1.8.7 any_instance block deprecation warnings.

Fixes #554.