Format js test via js linter

Hello, I thought I would pass this issue on as I use some code from this build in another project. Keep up the good work.

Error: ['id'] is better written in a dot notation.

var token = response['id'];

https://github.com/humanmade/wp-stripe/blob/master/js/wp-stripe.js#L32

Start at js/wp-stripe.js line 32 and review the linter error for response['id']. Update that expression to satisfy the linter, then run the JavaScript linter and confirm the warning is gone.

Written by the indexing model from the issue text.