Move source to dest only if source is newer.

def _move_update(sourcepath, destname):
    """Move source to dest only if source is newer."""
    if sys.platform.startswith('linux'):
        return os.system("/bin/mv -fu '%s' '%s'" % (sourcepath, destname))
    else:
        return os.system("rsync -ua --remove-source-files '%s' '%s'" % (sourcepath, destname))