[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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Merge pull request #3886 from rjackson/update_backburner

[BUGFIX beta] Update to latest Backburner.js.