Upgraded babel-eslint to v9.0.0-beta.2 with better support for Flow

Summary: Upgraded the library to fix an error in Babel causing wrong unused variable errors.

Differential Revision: D8765952

fbshipit-source-id: 352d1bb50ada76caed247bf9aa4521eff2d510e9

package.json

@@ -201,7 +201,7 @@
   "devDependencies": {
     "@babel/core": "7.0.0-beta.47",
     "async": "^2.4.0",
-    "babel-eslint": "8.2.1",
+    "babel-eslint": "9.0.0-beta.2",
     "babel-generator": "^6.26.0",
     "eslint": "5.0.1",
     "eslint-config-fb-strict": "22.1.0",