Due to the recent changes to the Travis CI platform (see [1]), we will
now use GitHub Actions to run the tests.

Reference: https://docs.github.com/en/free-pro-team@latest/actions/guides/building-and-testing-nodejs

[1]: https://blog.travis-ci.com/2020-11-02-travis-ci-new-billing

A WebSocket-only v3.x client is able to reach a v2.x server, as the
Engine.IO handshake are compatible, but the v2.x server does not send
a sid in the Socket.IO handshake, which resulted in:

> Uncaught TypeError: Cannot read property 'sid' of undefined

A 'connect_error' event will now be emitted.

References:

- https://github.com/socketio/engine.io-protocol#difference-between-v3-and-v4
- https://github.com/socketio/socket.io-protocol#difference-between-v5-and-v4
- https://socket.io/docs/v3/migrating-from-2-x-to-3-0/

When a given socket was disconnected, either by the server-side or by the client-side, the manager was closed too, regardless of the other connected sockets.

```js
const socket1 = io({
  autoConnect: false
});
const socket2 = io("/test");

socket1.disconnect(); // also disconnect socket2
```

This bug was introduced in [1].

[1]: b60e909

Diff: 3.0.3...3.0.4