ci: migrate to GitHub Actions

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

src/test/java/io/socket/engineio/client/SSLConnectionTest.java

@@ -52,7 +52,7 @@ public class SSLConnectionTest extends Connection {
         TrustManagerFactory tmf = TrustManagerFactory.getInstance("SunX509");
         tmf.init(ks);
 
-        SSLContext sslContext = SSLContext.getInstance("TLSv1");
+        SSLContext sslContext = SSLContext.getInstance("TLS");
         sslContext.init(kmf.getKeyManagers(), tmf.getTrustManagers(), null);
 
         sOkHttpClient = new OkHttpClient.Builder()