Bump helmet from 3.12.1 to 3.15.0

[dependabot-preview[bot]]

Bumps helmet from 3.12.1 to 3.15.0.

Changelog

*Sourced from [helmet's changelog](https://github.com/helmetjs/helmet/blob/master/HISTORY.md).* > 3.15.0 / 2018-11-07 > =================== > > - new: `featurePolicy` now supports four new features > > 3.14.0 / 2018-10-09 > =================== > > - new: `featurePolicy` middleware > > 3.13.0 / 2018-07-22 > =================== > > - new: `permittedCrossDomainPolicies` middleware > > 3.12.2 / 2018-07-20 > =================== > > - fix: remove `lodash.reduce` dependency from `csp`

Commits

- [`0ec5b6c`](https://github.com/helmetjs/helmet/commit/0ec5b6ce94e6da3006291b2713aa266fd28ae279) 3.15.0 - [`8c0b6bd`](https://github.com/helmetjs/helmet/commit/8c0b6bd044bf7c2bc0088484cf607f469c5dfd08) Prepare 3.15.0 release - [`d81dc40`](https://github.com/helmetjs/helmet/commit/d81dc40af8867a6c33ae51005dc1f6744bed2c06) 3.14.0 - [`c734b30`](https://github.com/helmetjs/helmet/commit/c734b30cde13345ac0a982dcb0e7cba819830888) Update changelog for 3.14.0 release - [`ebf80d7`](https://github.com/helmetjs/helmet/commit/ebf80d75e3d6e1d5b433f4534607ddb232122d51) Travis shouldn't test on EOL'd Node 4; should test on 8 - [`db8fdc7`](https://github.com/helmetjs/helmet/commit/db8fdc70fc2b1ddf935c8160fc2428e76baa0778) Add featurePolicy middleware - [`0468c37`](https://github.com/helmetjs/helmet/commit/0468c37555dc434c808aa61bc5684d52f49dbce5) Add mocha.opts to check for a few rare bugs - [`d75a4a3`](https://github.com/helmetjs/helmet/commit/d75a4a34002bcd66d06723b175c50105fd4e4e11) 3.13.0 - [`56fbf16`](https://github.com/helmetjs/helmet/commit/56fbf16c791576d1b459a19bfaae5b233bbeac5a) Update changelog for 3.13.0 release - [`5d0a9ef`](https://github.com/helmetjs/helmet/commit/5d0a9ef536e2c66a5b25d9e045aa07232ba604ac) Travis should test on Node 10 instead of Node 8 - Additional commits viewable in [compare view](https://github.com/helmetjs/helmet/compare/v3.12.1...v3.15.0)

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Superseded by #139.