Update dependency helmet to v3.15.1

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change	References
helmet	dependencies	minor	3.9.0 -> 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` ### [`v3.15.0`](https://togithub.com/helmetjs/helmet/blob/master/HISTORY.md#​3150--2018-11-07) [Compare Source](https://togithub.com/helmetjs/helmet/compare/v3.14.0...v3.15.0) =================== - new: `featurePolicy` now supports four new features ### [`v3.14.0`](https://togithub.com/helmetjs/helmet/blob/master/HISTORY.md#​3140--2018-10-09) [Compare Source](https://togithub.com/helmetjs/helmet/compare/v3.13.0...v3.14.0) =================== - new: `featurePolicy` middleware ### [`v3.13.0`](https://togithub.com/helmetjs/helmet/blob/master/HISTORY.md#​3130--2018-07-22) [Compare Source](https://togithub.com/helmetjs/helmet/compare/v3.12.2...v3.13.0) =================== - new: `permittedCrossDomainPolicies` middleware ### [`v3.12.2`](https://togithub.com/helmetjs/helmet/blob/master/HISTORY.md#​3122--2018-07-20) [Compare Source](https://togithub.com/helmetjs/helmet/compare/v3.12.1...v3.12.2) =================== - fix: remove `lodash.reduce` dependency from `csp` ### [`v3.12.1`](https://togithub.com/helmetjs/helmet/blob/master/HISTORY.md#​3121--2018-05-16) [Compare Source](https://togithub.com/helmetjs/helmet/compare/v3.12.0...v3.12.1) =================== - fix: `expectCt` should use comma instead of semicolon as delimiter ### [`v3.12.0`](https://togithub.com/helmetjs/helmet/blob/master/HISTORY.md#​3120--2018-03-02) [Compare Source](https://togithub.com/helmetjs/helmet/compare/v3.11.0...v3.12.0) =================== - new: `xssFilter` now supports `reportUri` option ### [`v3.11.0`](https://togithub.com/helmetjs/helmet/blob/master/HISTORY.md#​3110--2018-02-09) [Compare Source](https://togithub.com/helmetjs/helmet/compare/v3.10.0...v3.11.0) =================== - update: main Helmet middleware is now named to help with debugging ### [`v3.10.0`](https://togithub.com/helmetjs/helmet/blob/master/HISTORY.md#​3100--2018-01-23) [Compare Source](https://togithub.com/helmetjs/helmet/compare/v3.9.0...v3.10.0) =================== - new: `csp` now supports `prefix-src` directive - update: `csp` no longer loads JSON files internally, helping some module bundlers - fix: `false` should be able to disable a CSP directive

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