Publish builds to S3.

This will publish builds to S3 to the following URL's:

```
<S3_FILE_PREFIX>/ember-validations.js
<S3_FILE_PREFIX>/ember-validations.prod.js
<S3_FILE_PREFIX>/ember-validations.min.js
<S3_FILE_PREFIX>/daily/<YYYYMMDD>/ember-validations.js
<S3_FILE_PREFIX>/daily/<YYYYMMDD>/ember-validations.prod.js
<S3_FILE_PREFIX>/daily/<YYYYMMDD>/ember-validations.min.js
<S3_FILE_PREFIX>/shas/<SHA>/ember-validations.js
<S3_FILE_PREFIX>/shas/<SHA>/ember-validations.prod.js
<S3_FILE_PREFIX>/shas/<SHA>/ember-validations.min.js
```

The following secure environment values need to be added to
`.travis.yml`:

* S3_ACCESS_KEY_ID
* S3_SECRET_ACCESS_KEY
* S3_BUCKET_NAME - Pre-populated to `builds.dockyard.com`
* S3_FILE_PREFIX - Pre-populated to `ember-validations`

----

These can be added after merging this PR via the following commands:

```sh
gem install travis
travis encrypt S3_ACCESS_KEY_ID=some-value-here --add
travis encrypt S3_SECRET_ACCESS_KEY=some-value-here --add
```