Add eslint 6 support

[HDuck]

Fixed tests to check eslint ver.6. Added eslint-6 support.

[HDuck]

Check failes on Node.js 6

[graue]

This patch looks good. ESLint 6 no longer supports Node.js 6, so the CI build failing for Node 6 is expected. Probably need to update .travis.yml to install ESLint 5 when running on Node 6.

[platinumazure]

Hi everyone, I'm from ESLint.

Instead of doing a direct require to a formatter path, I would recommend CLIEngine#getFormatter (if you have a CLIEngine instance) or CLIEngine.getFormatter (static function you can use if you don't have an instance).

Requiring the path directly is not part of the public API and could change any time, even in patch releases. The two CLIEngine options are in the public API and won't change except in a major release (and then, only when absolutely needed and we can't deprecate).

Good luck! Please stop by our Gitter chat if you need help.

[lochstar]

@HDuck See #238 (comment) regarding the recommended approach for the formatter options.

[HDuck]

I'll fix it all today. Thanks for assisting. If anyone knows how to change travis config to install eslint 5 on node 6 I'll appreciate additional help. ;)

[Sunshine168]

@HDuck I have a suggestion which drop node 6 in travis.yml file and add node engines require in package.json { "engines" : { "node" : "> 6" } } ,then update the doc. Because for Eslint 6 , Node.js 6 is no longer supported it .

[vinayakkulkarni]

Fixes #271

This PR would fix the stylish formatter issue

[alexander-akait]

Ping me when the PR was ready for review

[HDuck]

@evilebottnawi ready.

[alexander-akait]

Two changes. One note. Also do you test this code with eslint@5? We doesn't have CI tests on this case, so need manual testing. Thanks for PR.

[HDuck]

@evilebottnawi Tested manually eslint@^4 and eslint@^5 versions. All is okay.

[lochstar]

Should the README be updated to reflect the new usage?