Update dependency helmet to v3.15.1

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change	References
helmet	dependencies	patch	3.15.0 -> 3.15.1	homepage, source

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Release Notes

helmetjs/helmet

### [`v3.15.1`](https://togithub.com/helmetjs/helmet/blob/master/HISTORY.md#​3151--2019-02-10) [Compare Source](https://togithub.com/helmetjs/helmet/compare/v3.15.0...v3.15.1) =================== - update: deprecate `hpkp`

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