Update dependency mongoose to v5.4.18

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change	References
mongoose	dependencies	patch	5.4.17 -> 5.4.18	homepage, source

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Release Notes

Automattic/mongoose

### [`v5.4.18`](https://togithub.com/Automattic/mongoose/blob/master/History.md#​5418--2019-03-08) [Compare Source](https://togithub.com/Automattic/mongoose/compare/5.4.17...5.4.18) =================== - fix(document): handle nested virtuals in populated docs when parent path is projected out [#​7491](https://togithub.com/Automattic/mongoose/issues/7491) - fix(model): make subclassed models handle discriminators correctly [#​7547](https://togithub.com/Automattic/mongoose/issues/7547) - fix(model): remove $versionError from save options for better debug output [#​7570](https://togithub.com/Automattic/mongoose/issues/7570)

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