Reference and build from ebryn/backburner.js and remove amd

Tagged the `bower.json` to the last version of backburner as per https://github.com/emberjs/ember.js/pull/4926.  

This should make it considerably easier to upgrade backburner as well as play into future plans for using the bundled output for the es6 module transpiler (by having all inputs in es6).  [discussed with @rwjblue]

Assuming this works well, we'll move forward with this on RSVP and other vendored packages.