issue with strings in old gem

not quite sure, but james said to add here.

full details: https://secure.helpscout.net/conversation/54954946/19802/

```
trackable :id, :email, {extra: vero_properties}
def vero_properties
{
first_name: first_name.to_s
}
end
```

Instead of:

```
trackable :id, :email, :first_name
```
