Add @babel/core to RN dev deps for testing

Summary:
This adds a locked down version of Babel 7 (beta.40) to the dev dependencies for RN because it's used in testing.

See also; f8d6b97140 (r28569573)

Reviewed By: davidaurelio

Differential Revision: D7685590

fbshipit-source-id: eae8379b586d82527788e9812a573ab321dfc8e3

package.json

@@ -198,6 +198,7 @@
     "yargs": "^9.0.0"
   },
   "devDependencies": {
+    "@babel/core": "7.0.0-beta.40",
     "babel-eslint": "8.2.1",
     "eslint": "4.17.0",
     "eslint-config-fb-strict": "22.1.0",