Don't require full output of a deb package if we're just checking for
tar flags.

The reason for this is that if we do output, it fails like so;

  1) FPM::Package::Deb#tar_flags should set the group for the package's data files
     Failure/Error: @package.output(@target)
     FPM::Util::ProcessFailed:
       tar failed (exit code 2). Full command was:["tar", "-C", "/tmp/package-deb-staging20140423-5427-10uiv96", "-z", "--numeric-owner", "--owner", "0", "--group", "nogroup", "-cf", "/tmp/package-deb-build20140423-5427-19uvbmw/data.tar.gz", "."]

Because 'nogroup' isn't a valid group when tar is run, but we want to check
that 'nogroup' is set in the tar args, not that tar works ;)