Make `yield_control` default to `at_least(:once)`.

Previously, it raised a confusing error when multiple
yields occurred:

> The yield_control matcher is not designed to be used with a
> method that yields multiple times. Use the yield_successive_args
> matcher for that case.

This was confusing because yield_control has supported multiple
yields for awhile.  This also simplifies the implementation!
Merge pull request #675 from rspec/improve-yield-control-matcher

Make `yield_control` default to `at_least(:once)`.