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