Update dependency gulp-eslint to v6

[renovate]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
gulp-eslint	^1.1.1 → ^6.0.0

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Release Notes

adametry/gulp-eslint (gulp-eslint)

v6.0.0

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• 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

v5.0.0

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• Bump eslint dependency to ^5.0.0 https://eslint.org/blog/2018/06/eslint-v5.0.0-released
• Use destructuring assignment to simplify the code

v4.0.2

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• Update plugin-error to ^1.0.0

v4.0.1

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• Make fix option work even if quiet option is also enabled
• Remove deprecated gulp-util dependency and use individual modules instead

v4.0.0

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• Drop support for linting Stream contents
  • Because almost all, at least widely used tools to handle JavaScript files don't support Streams. They only support either String or Buffer.
• Use Buffer.from(<string>) instead of the deprecated new Buffer(<string>)
  • Note that Buffer.from is only available on Node.js >= 4.5.0.
• Bump eslint dependency to ^4.0.0
• Emit a PluginError when it fails to load an ESLint plugin

v3.0.1

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• Remove unnecessary object-assign dependency

v3.0.0

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• Bump eslint dependency to ^3.0.0 https://eslint.org/blog/2016/07/eslint-v3.0.0-released
• Use ES2015 syntax
• Remove these deprecated option aliases:
  • global
  • env
  • config
  • rulesdir
  • eslintrc
• Drop support for non-array globals option

v2.1.0

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• Remove now obsolete error handling for formatter loading
  • It's gracefully done in ESLint >=v2.10.0. eslint/eslint#5978

v2.0.0

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• Update to ESLint 2.0.0, along with other dependency updates
• Replace JSCS with ESLint equivalent rules

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