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

Note: the certificate was updated because it failed with newer Node.js versions

```
_tls_common.js:129
      c.context.setCert(cert);
                ^

Error: error:140AB18F:SSL routines:SSL_CTX_use_certificate:ee key too small
    at Object.createSecureContext (_tls_common.js:129:17)
    at Server.setSecureContext (_tls_wrap.js:1328:27)
```

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

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

This fixes NullPointerException when response doesn't have content type. Exception stack trace:

```
java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String okhttp3.MediaType.toString()' on a null object reference
        at io.socket.engineio.client.transports.PollingXHR$Request.onLoad(PollingXHR.java:271)
        at io.socket.engineio.client.transports.PollingXHR$Request.access$700(PollingXHR.java:148)
        at io.socket.engineio.client.transports.PollingXHR$Request$1.onResponse(PollingXHR.java:232)
        at okhttp3.RealCall$AsyncCall.execute(RealCall.java:141)
        at okhttp3.internal.NamedRunnable.run(NamedRunnable.java:32)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1162)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:636)
        at java.lang.Thread.run(Thread.java:764)
```

It was already present in the Travis test matrix.

Versions above 3.12.x have dropped support for Java 7.

Related: https://square.github.io/okhttp/changelog_3x/#version-3130

Reference: https://github.com/socketio/engine.io-protocol#difference-between-v3-and-v4

Similar to the option of the JS client:

```java
opts = new Socket.Options();
opts.extraHeaders = singletonMap("authorization", singletonList("bearer abcd"));
socket = new Socket(opts);
```

Note: the refactor of the options (similar to [1]) will be done in a
future step.

[1] socketio/engine.io-client@5f47a50

Before this fix, the client could mark the polling transport as open
even though the handshake packet was not received properly (for
example, after a parsing error).

See also: socketio/engine.io-client@1c8cba8

…cketio#106)

Co-authored-by: Pavel Valodzka <pavel@valodzka.name>

…ketio#110)

Bumps [engine.io](https://github.com/socketio/engine.io) from 4.0.5 to 4.1.2.
- [Release notes](https://github.com/socketio/engine.io/releases)
- [Changelog](https://github.com/socketio/engine.io/blob/4.1.2/CHANGELOG.md)
- [Commits](socketio/engine.io@4.0.5...4.1.2)

---
updated-dependencies:
- dependency-name: engine.io
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>