Add node versions 4, 5, 6, and latest stable to the travis tests
Add node versions 4, 5, 6, and latest stable to the travis tests

- With #7 and tests depending on external configuration (specifclly certificates) fixed, tests can target more versions.
- This commit adds node versions 4, 5, 6 and the latest stable to the travis test configurations.

getdnsapi/getdns-node,joelpurra/getdns-node,joelpurra/getdns-node,getdnsapi/getdns-node,joelpurra/getdns-node,getdnsapi/getdns-node