Ensuring exit status is always set, even when $? isn't

Hi,

For an app I'm building ontop of cocaine, I'd like to be able to use the `FakeRunner` for testing in certain circumstances (for instance to log what commands will be run w/o actually running them). However, when I attempt to, the `$?` object is nil, and thus I get the following error:

```
/Users/drj/.rvm/gems/ruby-2.0.0-p247/gems/cocaine-0.5.3/lib/cocaine/command_line.rb:91:in `run': Command '<my command>' returned . Expected 0 (Cocaine::ExitStatusError)
```

It would seem that setting `@exit_status` conditionally, as you were, is unsafe since it is possible it will not be set. I have made the following (very small) change to set it to 0 when `$?` is unavailable. This means when using the FakeRunner (or if for some reason `$?` is not supported by the given runner), the exit status checking won't throw a fit.