Fix travis.yml to run with correct Node.js version

This changes the `.travis.yml` to use the correct Node.js version
by using strings as version numbers.

Relevant Travis CI documentation page: http://about.travis-ci.org/docs/user/languages/javascript-with-nodejs/#Choosing-Node-versions-to-test-against

Without using strings to specify the desired versions Travis CI
runs on Node.js 0.1 instead of 0.10.