fix 'TypeError: can't convert Symbol into Integer' if multiple leads are returned in Rapleaf::Marketo::Client#get_lead

Stack trace:

```text
/gems/marketo-1.4.0/lib/marketo/lead_record.rb:13 in "[]"
/gems/marketo-1.4.0/lib/marketo/lead_record.rb:13 in "from_hash"
/gems/marketo-1.4.0/lib/marketo/client.rb:180 in "get_lead"
/gems/marketo-1.4.0/lib/marketo/client.rb:73 in "get_lead_by_email"
```