Upgraded Babel to v7.0.0-beta.48

Summary:
This is the same version we're using on www. It has support for new Flow syntax and we need it on fbsource too.

allow_many_files
allow-large-files

Differential Revision: D8819980

fbshipit-source-id: 736d6d93ad879311e638da00873ca2acf42d1422

package.json

@@ -199,7 +199,7 @@
     "yargs": "^9.0.0"
   },
   "devDependencies": {
-    "@babel/core": "7.0.0-beta.47",
+    "@babel/core": "7.0.0-beta.48",
     "async": "^2.4.0",
     "babel-eslint": "9.0.0-beta.2",
     "babel-generator": "^6.26.0",