Use existing s3 credentials from Travis.yml

https://github.com/tildeio/rsvp.js/commit/ea2263764439df4569ff4efd9a80c87ccdb3184a Added grunt s3 integration.  You'll notice that in this commit there are two encrypted variables that were added to the `.travis.yml` file.  The default variable names for `grunt-s3` can be found [here](https://github.com/pifantastic/grunt-s3#environment-variables).  

I've simply set the `ember-publisher` credentials to the ones we know exist from our exploration (see above).

Cheers