I have done some more testing with the TCP-based join configuration (using CentOS and Windows, but not Ubuntu), and I can confirm that the <tcp-ip> child elements should be <member> and not <hostname>. This is a change that the Hazelcast implementation made a few versions ago. As I mentioned earlier, you can refer to the documentation for more information - it appears they have recently updated the documentation as well.
As an aside, it appears that the Hazelcast team has pulled the 3.1.5 release from their list of "prior releases", possibly due to issues with that version. I am in the process of updating the plugin to use the latest 3.1.x release (currently 3.1.7). While I do not know if this will have an effect for your particular situation, it might be worth a shot. You can update your own copy in-place, or wait for the next nightly build to pick up the changes from the repository.