Stop recommending bluebird's own event

[benjamingr]

Bluebird has emitted the window unhandled rejection event in addition to having an onPossiblyUnhandledRejection hook for around 10 years at this point..

[netlify]

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[jennifer-shehane]

@benjamingr Thanks for the contribution! Could you please sign our CLA?

[jennifer-shehane]

Closing as the CLA is required to be signed before merging changes.