Test against all supported Java versions

.github/workflows/ci.yml

@@ -1,9 +1,21 @@
-name: Build & Test
-on: [push, pull_request]
+name: CI
 
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+    branches:
+      - master
 jobs:
   build:
+    name: Test on Java ${{ matrix.java }}
     runs-on: ubuntu-latest
+
+    strategy:
+      matrix:
+        java: [8, 9, 10, 11, 12, 13, 14, 15]
+
     steps:
       - name: Checkout
         uses: actions/checkout@v2
@@ -11,7 +23,7 @@ jobs:
       - name: Set up JDK 1.8
         uses: actions/setup-java@v1
         with:
-          java-version: 8
+          java-version: ${{ matrix.java }}
 
       - name: Cache Maven packages
         uses: actions/cache@v2