Travis CI should also test in newer versions of Node.js

Node.js version `0.6` listed in `.travis.yml` is slightly outdated, version 0.10 is currently the latest stable.

I've added `0.10` and `0.8` to test in, and also quoted them (`0.6`→`"0.6"`) as in the corresponding [example](http://about.travis-ci.org/docs/user/languages/javascript-with-nodejs/#Choosing-Node-versions-to-test-against).