Fixed incorrect exception thrown for missing EM

The `Client#em_http_client` method checks whether EventMachine is available. However, the `#send_async` method tries to create an instance of `EM::DefaultDeferrable` before this check, which causes `NameError: uninitialized constant Pusher::Request::EM` instead of expected exception `Pusher::Error: In order to use async calling you must be running inside an eventmachine loop`.