add GitHub Actions CI

This runs the tests on every PR and branch push (including master
merges). Handy for avoiding dumb compile bugs and validating changes.

.github/workflows/test.yml

@@ -0,0 +1,16 @@
+name: Service CI
+
+on: [push, pull_request]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v2
+      - name: Set up JDK 11
+        uses: actions/setup-java@v1
+        with:
+          java-version: 11
+      - name: Build with Maven
+        run: mvn -e -B package