Removing explicit checking of object's private methods

This PR changes predicate matcher behavior to no longer inspect an object's private methods using a call to `private_methods`. Calling `respond_to?(:method, false)` should exclude private methods from searching. This does reduce verbosity when reporting on expectation failures; however, this will correct an issue by which RSpec causes side-effect errors when trying to call a predicate.
It would be preferable to restore this behaviour, this matcher used to work with private methods and this serves to help those who relied upon the old meaning.