the json that tokenInput is waiting for must have these two attributes: 'id' and 'name'
so, you have two choices, either you modify your @postcodes elements to add these attributes to each one or you could change the attributes names used as key and value for elements in your coffeescript like this
$('#postcode_tokens').tokenInput '/postcodes.json', {
tokenValue: "yourCustomId",
propertyToSearch; "yourCustomName"
}