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

.github/workflows/ci.yml

@@ -0,0 +1,34 @@
+name: CI
+
+on:
+  push:
+  pull_request:
+  schedule:
+    - cron: '0 0 * * 0'
+
+jobs:
+  verify:
+    runs-on: ubuntu-latest
+
+    strategy:
+      matrix:
+        java: [8, 11]
+
+    steps:
+      - uses: actions/checkout@v2
+      - name: Setup java
+        uses: actions/setup-java@v1
+        with:
+          java-version: ${{ matrix.java }}
+      - name: Cache Maven packages
+        uses: actions/cache@v2
+        with:
+          path: ~/.m2
+          key: ${{ runner.os }}-m2-${{ hashFiles('**/pom.xml') }}
+          restore-keys: ${{ runner.os }}-m2
+      - name: Setup Node.js
+        uses: actions/setup-node@v1
+        with:
+          node-version: 14.x
+      - name: Run the Maven verify phase
+        run: mvn verify -Dgpg.skip=true