[BUGFIX beta] Update backburner.

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https://github.com/ebryn/backburner.js/compare/a7dcf586bc840021d7a585eabf9fd631d76c0c35...24ff580bef0f2d402474cc3d221da73ac82f09c9
Merge pull request #5676 from rwjblue/update-backburner

[BUGFIX beta] Update backburner.