build: update devdependency generate-changelog to ~1.8.0

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
generate-changelog	devDependencies	minor	~1.7.0 -> ~1.8.0

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Release Notes

lob/generate-changelog

### [`v1.8.0`](https://togithub.com/lob/generate-changelog/blob/master/CHANGELOG.md#​180-2019-06-25) [Compare Source](https://togithub.com/lob/generate-changelog/compare/v1.7.1...v1.8.0) ##### Documentation Changes - **readme:** mention commit ranges ([#​43](https://togithub.com/lob/generate-changelog/pull/43)) ([b4a75c49](https://togithub.com/lob/generate-changelog/commit/b4a75c49c66b265e768c8d6477a409b5e722b8da)) ##### New Features - Allow unknown commit types ([#​46](https://togithub.com/lob/generate-changelog/pull/46)) ([b2fb9ff5](https://togithub.com/lob/generate-changelog/commit/b2fb9ff5517b7be309f6050dd7b557b53668e56b)) ##### 1.7.1 (2018-03-28) ##### Bug Fixes - **writer:** fix commit URL for Gitlab repos ([#​40](https://togithub.com/lob/generate-changelog/pull/40)) ([656a2bcc](https://togithub.com/lob/generate-changelog/commit/656a2bcc589433cf7f9982e5ca4d0493a56126eb))

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