Upgrade to babel@7.0.0

Summary: Now that babel7 is stable, we can upgrade Metro and fbsource to use it, yay!!!!!

Reviewed By: mjesun

Differential Revision: D9518571

fbshipit-source-id: c85569cb3058235f4f9310949897f7955ecf7324

package.json

@@ -202,7 +202,7 @@
     "yargs": "^9.0.0"
   },
   "devDependencies": {
-    "@babel/core": "7.0.0-beta.56",
+    "@babel/core": "^7.0.0",
     "async": "^2.4.0",
     "babel-eslint": "9.0.0-beta.2",
     "babel-generator": "^6.26.0",