fix update server info verb

Hi,

When trying to update server info it fails :

```ruby
Postmark::ApiClient.new(my_api_key).update_server_info inbound_domain: 'my.domain.com'
Postmark::UnknownError: #<Net::HTTPNotFound:0x007fcf23c66ca0>
```

Postmark changed their API from POST to PUT HTTP verb.