Enable getter-return rule as a warning (facebook#3723)

* Updating ESlint to ^4.15.0 and adding new rules to config

* remoning style rule and auto fixing breakages from new rules

* Removing implicit-arrow-linebreak style rule

* adding new rule to eslint config project

* updating react scripts eslint version

* Pinning version.

* Changing getter-return to warn

* Update package.json

* Update .eslintrc

Author: chrislaughlin

Diff for: packages/eslint-config-react-app/index.js

@@ -238,6 +238,7 @@ module.exports = {
           'Please use import() instead. More info: https://github.com/facebookincubator/create-react-app/blob/master/packages/react-scripts/template/README.md#code-splitting',
       },
     ],
+    'getter-return': 'warn',
 
     // https://github.com/benmosher/eslint-plugin-import/tree/master/docs/rules
     'import/first': 'error',

Diff for: packages/react-scripts/package.json

@@ -33,8 +33,8 @@
     "chalk": "1.1.3",
     "css-loader": "0.28.7",
     "dotenv": "4.0.0",
-    "dotenv-expand": "4.2.0",
-    "eslint": "4.10.0",
+    "dotenv-expand": "4.0.1",
+    "eslint": "4.15.0",
     "eslint-config-react-app": "^2.1.0",
     "eslint-loader": "1.9.0",
     "eslint-plugin-flowtype": "2.39.1",