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

package.json

@@ -34,7 +34,7 @@
     "css-loader": "0.28.7",
     "dotenv": "4.0.0",
     "dotenv-expand": "4.0.1",
-    "eslint": "4.10.0",
+    "eslint": "4.15.0",
     "eslint-config-react-app": "^2.1.0",
     "eslint-loader": "1.9.0",
     "eslint-plugin-flowtype": "2.39.1",