Use user.id as CustomerId because emails can go over the 50 char limit

A common occurrence of this is when using `spree_social` and the user opts not to share their email, facebook creates a proxy email address that is often over 50 characters long. ShipStation will reject this as the character limit on CustomerID is 50, although for email it is 100. I think user.id is the right field to send as a unique customer ID anyway...