Remove Node.js 0.6 from Travis testing.
Travis has been having issues with testing Yeti under Node.js 0.6. I have not been able to reproduce these problems outside of Travis. Since these failures are likely false positives, 0.6 testing is now removed. For now, 0.6 remains a supported runtime and I will continue to test on 0.6 before releases and pull request merges. The problem may be related to node-portfinder's behavior on Travis, because failures began around 4afa448. Ref #115.