automatically publish passing master branch builds to s3

This commit will publish passing builds of the master branch to s3 for
convenient downloading.

Files created if the build is successful:
ember-data-latest.js
ember-data-latest.min.js
ember-data-\<revision\>.js
ember-data-\<revision\>.min.js

Any *-latest*.js files will be overwritten to the build that passes and
runs `rake publish_build`.

To add credentials to your travis instance:

gem install travis
travis encrypt S3_ACCESS_KEY_ID=yourkey --add
travis encrypt S3_SECRET_ACCESS_KEY=yourkey --add

You'll also need to update the S3_BUCKET_NAME=yourbucketname in
.travis.yml