async updateCustomerEmail(id, email) {
        debug(`updateCustomerEmail(${id}, ${email})`);
        try {
            await this._rateLimitBucket.throttle();
            const customer = await this._stripe.customers.update(id, {email});
            debug(`updateCustomerEmail(${id}, ${email}) -> Success`);
            return customer;
        } catch (err) {
            debug(`updateCustomerEmail(${id}, ${email}) -> ${err.type}`);
            throw err;
        }
    }
Update the email address for a Stripe Customer.

@param {string} id
@param {string} email

@returns {Promise<ICustomer>}