Add flag to print output during long builds for Travis CI

Created by: ollieparanoid

Travis will kill the job, if there's no output for 10 minutes. For #64 (closed) we need to build packages in pmbootstrap, which will not have any output for that long. It would be useful to have a flag, that when enabled, prints out a dot every minute or so to let Travis know, that pmbootstrap is still doing what it should.