Add --dry-run as an argument

Sometimes when working on a collector we would like to be able to see
what is going to be submitted to backdrop but not actually execute the
POST. I've added an argument to `backdrop_collector` so that you can
cause backdrop.collector.write.DataSet.post to output to the logs rather
than executing its POST.
Update .travis.yml for new deps

I forgot to change how travis ran the tests after updating how the deps
are install and the tests run. Our travis.yml now matches the structure.