Update Published S3 Asset Structure

This PR changes the publishing structure for [builds.emberjs.com](http://builds.emberjs.com) to the following:

```
# the following will be written/generated everytime EmberDev::Publish.to_s3 is called:
builds.emberjs.com/shas/<sha>/<files>
builds.emberjs.com/tags/v1.0.0/<files>
builds.emberjs.com/latest/<files>
builds.emberjs.com/daily/<YYYYMMDD>/<files>

# the following will be written/generated only when EmberDev::Publish.to_s3 is called with the :stable => true option:
builds.emberjs.com/stable/<files>
```