Run tests with Node.js 7 on Travis as well

[nstepien]

I think I caught some rdkafka error last night when trying to kill a process running with Node 7. Could be completely unrelated.
Either way testing on Node 7 is a good thing.

[webmakersteve]

Hi there:

First I'd like to say thank you very much for your interest in node-rdkafka! Glad to see you're using it on node 7, and I definitely hope it's working well for you.

Unfortunately, while we're happy and willing to accept contributions to fix issues specific to node 7 (provided they do not degrade the experience of our officially supported node versions), adding it to our CI tests would mean we would have to officially support it. We have elected only to support this package on LTS versions of node (i.e., currently, 4 and 6).

With that said, I can't accept the PR at this time. Please feel free to continue reporting issues if they arise as it helps track problems with the project and give insight to others using it.

Thank you :)