Replace TravisCI with GitHub Actions (#995)

* Replace TravisCI with GitHub Actions

As of Nov 2020, TravisCI has given open source projects on their
"Free" plan 10k build credits. These are non-renewable, and we have
already used our entire allotment.

While TravisCI has suggested additional build credits can be awarded
to OSS projects by manually contacting them, this is unfortunately a
high energy barrier for continuing to use their service.

The GitHub Actions ecosystem has become very robust over the past
year, and we can achieve the same CI workflow (and more) by switching
to GitHub Actions.

We checkout the repo with `fetch-depth: 2` to fix a Codecov warning:

> Issue detecting commit SHA. Please run actions/checkout with fetch-depth > 1 or set to 0

.github/workflows/run-tests.yml

@@ -0,0 +1,26 @@
+name: Run Tests
+
+on:
+  # Run tests for any PRs.
+  pull_request:
+
+jobs:
+  build-and-test:
+    runs-on: ubuntu-20.04
+
+    steps:
+      - name: Checkout the source code
+        uses: actions/checkout@v2
+        with:
+          # Needed by Codecov
+          fetch-depth: 2
+
+      - name: Run tests
+        run: |
+          composer run start:integration-services
+          composer run test
+
+      - name: Upload coverage to Codecov
+        uses: codecov/codecov-action@v1
+        with:
+          files: test/coverage.xml