Can't parse JSON when too many users

I got a error as follow.
```
 (JSON::ParserError)
	from /usr/local/Cellar/ruby/2.4.1_1/lib/ruby/2.4.0/json/common.rb:156:in `parse'
	from export.rb:88:in `save_json'
	from export.rb:19:in `save_accepted_invites'
	from export.rb:95:in `<main>'
```

I guess that it was caused by very long user data.