GitVersionApp should set exit code

[gitfool]

Since switching GitVersion to use generic hosting (17bfb2e), GitVersion stopped returning an exit code. This leads to ugliness with tools like Cake that try to parse the non-existent json output.

[arturcic]

@gitfool do you have a fix for this?

[gitfool]

@arturcic I think so. Not completely sure, but then I can't see any other way to set the exit code.