Fixes #18: Add .travis.yml file

The .travis.yml file passes `travis-lint`:

```bash
> bundle exec travis-lint
Hooray, /Users/xgjo/src/chef-vault/.travis.yml seems to be solid!
```

Kevin,

You'll need to follow steps 1, 2, and 4 in this [Travis CI Getting Started Guide](http://about.travis-ci.org/docs/user/getting-started/)