Don't allow at_least(0)
Created by: dchelimsky
See #132 (closed) for background.
This statement doesn't really test anything:
object.should_receive(:message).at_least(0).times
It will pass if the object receives the message any number of times, or never really receives it. It is really just a stub.
Let's raise an error in 3.0 recommending the user use stub
instead, and add a deprecation warning in a 2.x release with the same message.