ignore babelrc in git-upgrade (facebook#20790)

Summary:
Per [this comment on the issue](facebook#20710 (comment)):

> Babel by default tries to find .babelrc files to configure itself, and once it finds the one from the RN app (which links to the RN preset that only works on Babel 7) it fails.
> The easiest patch to fix this issue is to add babelrc: false to the babel-register method, so Babel does not read any additional .babelrc file.

Fixes facebook#20710
Pull Request resolved: facebook#20790

Differential Revision: D9458502

Pulled By: hramos

fbshipit-source-id: 3a0ed7261322fdd9e0c0840f8a3944974f38b233

Author: ngraef

react-native-git-upgrade/cli.js

@@ -10,6 +10,7 @@
 'use strict';
 
 require('babel-register')({
+  babelrc: false,
   presets: [
     require('babel-preset-es2015-node'),
     require('babel-preset-stage-3'),