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

Compare Source

• Bump eslint dependency to ^6.0.0 https://eslint.org/blog/2019/06/eslint-v6.0.0-released
• Drop support for Node 6 and 7

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