Publish to s3

This changes the url that backburner was previously posted to from:

http://builds.emberjs.com.s3.amazonaws.com/backburner.js/lastest/backburner.js

to

http://builds.emberjs.com.s3.amazonaws.com/backburner.js/backburner-latest.js
AND
http://builds.emberjs.com.s3.amazonaws.com/backburner.js/lastest/backburner-min.js

This is consistent with RSVP's style which is the most closely related
micro lib (as far as build is concerned) to backburner.  If this is
undesired we can add the other urls

__IMPORTANT__

In order for the publishing to work the following variables have to be
set

- S3_BUCKET_NAME
- S3_SECRET_ACCESS_KEY
- S3_ACCESS_KEY_ID

inside the .travis.yml.  I am unable to set this at the moment, so this
is something that should be kept in mind during merge.