ignore PRs that aren't Open
    
    Sometimes it is necessary recreate the Jenkins PR polling job, or
    cleanup old runs. This can cause Jenkins to lose the record of which
    PRs it has already seen, and retest old PRs which are long closed.
    This change causes the polling job to check the state of the PR, and
    ignore it if it isn't Open. Testing PRs which are in any other state
    doesn't provide any value.