Revert "[BUGFIX beta] Upgrade Backburner to 0.2.0"

This reverts commit 736effe8343d9fc76447a6584895520e663bb97d.
Revert "[BUGFIX beta] Ember.onerror now uses Backburner's error handler - #3401"

This reverts commit 022c1f624e1a8e010f7134378f991f90c23ac523.
Merge pull request #4565 from rjackson/revert-backburner-update

Revert Backburner update due to Phantom Failures