Upgrade to ESLint 6

[ehmicky]

To upgrade to eslint 6 the following had to be changed:

• ESLint 6 drops support Node for 6, so I updated .travis.yml to only run for Node >=8.10.0.
• linter.options.ignore -> linter.isPathIgnored(filePath)
• unknown plugin error message changed, so one test had to be updated
• tests use file paths internal to eslint (for the formatters). Those had to be updated.

[coveralls]

Coverage remained the same at 99.31% when pulling f079235 on ehmicky:feature/eslint-v6 into 40d004f on adametry:master.

[ehmicky]

Tests are now passing on CI.

@shinnn

[shinnn]

@ehmicky Would you be interested in joining the project as a maintainer? I’ve been impressed with your continuous contributions on execa, and unfortunately I don’t have enough motivation to maintain this project as I personally no longer use gulp.

[shinnn]

It would be helpful if you update CHANGELOG.md in this PR. We can do it later, though.

[ehmicky]

I've fixed the CHANGELOG.

Thanks for the invitation (and your mention of my work on execa)! I am very interested in both ESLint and Gulp. However right now I am working on too many projects at the same time, so I'm not sure whether I should add another one to maintain. Could this invitation extend to the future, where I might have fewer projects to work on?

[shinnn]

Could this invitation extend to the future, where I might have fewer projects to work on?

Sure. I appreciate your kind response!

For now I’m merging this and will publish a new version by myself.

[shinnn]

https://github.com/adametry/gulp-eslint/releases/tag/v6.0.0