You can build your program in multiple platforms if multi os feature is enabled. As of now, only linux and osx are supported but the travis developer is getting to support freebsd in the near term (which I hope a lot).
But you need to send e-mail to support team to ask to enable it. In case of nim-fuse, it was only one day to receive an ack from Konstantin.
This feature needs to be enabled manually. If you wish to take advantage of this feature, please send email to email@example.com, indicating which repository you would like to have it enabled.