Update dependency gulp-eslint to v6

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
gulp-eslint	devDependencies	major	5.0.0 -> 6.0.0

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Release Notes

adametry/gulp-eslint

### [`v6.0.0`](https://togithub.com/adametry/gulp-eslint/blob/master/CHANGELOG.md#​600) [Compare Source](https://togithub.com/adametry/gulp-eslint/compare/v5.0.0...v6.0.0) - Bump `eslint` dependency to ^6.0.0 - Drop support for Node 6 and 7

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