[BUGFIX beta] Update to latest Backburner.js.

From https://github.com/ebryn/backburner.js/pull/54:

    Fix a bug where debounced item in index 0 isn't
    removed unless debounce for that target/method is called again, but
    then it's in index 0 and then isn't removed

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[BUGFIX beta] Ensure that each branch is bundled before building.

This resolves an issue that occurs when the `Gemfile.lock` differs
between branches.
Merge pull request #3887 from rjackson/update_ember-dev

[BUGFIX beta] Ensure that each branch is bundled before building.
Merge pull request #3886 from rjackson/update_backburner

[BUGFIX beta] Update to latest Backburner.js.