Added .travis.yml

Hi @paulrouget,

as long as firefox.html isn't inside an organisation it looks like only the owner of the repository itself can enable Travis. You need to [login](https://travis-ci.org/) once and than activate Travis for this repo [here](https://travis-ci.org/profile). After that it should be safe to merge this pull request.

Note: Travis automatically calls `$ npm test` as this is the canonical way for running tests in Node. Therefor we only need to tell Travis which language (and version) we use.