Change how build is resolved

This would fix i-m-c/jenkins-inheritance-plugin#14 allowing promoted builds to be inherited.  Because the promotion parent isn't the one that built the build ends up hanging indefinetely and breaks any future builds for the child job.  

The only caveat that I have is that I am not sure if keeping the project reference is the right way to do this, or if there is a better way of keeping a reference to the build project.