feat(eslint-config): upgrade eslint to v6

BREAKING CHANGE: We require eslint version 6.0 as a peer dependency now.
To upgrade your project using our eslint-config, bump up eslint version
in your package.json file to "^6.0.0".

The new eslint version added new recommended rules, most notably
"require-atomic-updates" and "no-prototype-builtins". You may get new
linting errors after upgrade, fix them by changing your code or adding
eslint-ignore comments as needed.

Author: bajtos

packages/eslint-config/package.json

@@ -12,7 +12,7 @@
   "peerDependencies": {
     "@typescript-eslint/eslint-plugin": "^1.11.0",
     "@typescript-eslint/parser": "^1.11.0",
-    "eslint": "^5.16.0",
+    "eslint": "^6.0.0",
     "eslint-config-prettier": "^5.0.0",
     "eslint-plugin-eslint-plugin": "^2.1.0",
     "eslint-plugin-mocha": "^5.3.0"