fix warnings about expectations of :success? on nil

It's not right to mock $?.success?, because $? can be nil at the
time the expectation is set, resulting in errors like:

    An expectation of :success? was set on nil. Called from /home/adam/.GIT/3rd-party/capistrano-unicorn/spec/config_spec.rb:36:in `block

Instead we simply ensure that $? is set to an appropriate value.

This may or may not fix the failures that some people have been seeing
(mentioned in #70).
Merge pull request #76 from aspiers/exit-status-warnings

fix warnings about expectations of :success? on nil