ignore babelrc in git-upgrade

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

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'),