Migrate s3 publishing to node.js

This effectively transitions the dependency on [ EmberDev::Publish.to_s3](https://github.com/emberjs/ember-dev/blob/master/lib/ember-dev/publish.rb) to the `lib/s3_publisher.js`.  The eventual goal is to remove the need to run `bundle install` from the `.travis.yml` which will quicken build times.

## Configuration

The following environment variables need to exist for the build to be published to s3

export S3_ACCESS_KEY_ID
export S3_SECRET_ACCESS_KEY
export S3_BUCKET_NAME (this is fine as is)

These are likely already specified in the secure keys of the `.travis.yml`, but I am unsure if it is exact (and as such should probably be updated.

Any feedback would be great! :))

Would love to pair on merging this if you decide to merge.  Thanks!
This is likely fine, but throwing here will mean the process exists with a non-zero exit code, and the bower publishing won't happen.  Most likely, this will be happening when no credentials exist, which means that we are running on a PR, so this is probably fine.  Can you confirm that is the intent?