Add Github Actions workflow gradle.yml

Use Github Actions built-in Gradle 5.6.2

Github Actions virtual environments all include
a pre-installed Gradle that is 5.6 or later, so
we can just build with it without using the wrapper.

.github/workflow/gradle.yml

@@ -0,0 +1,22 @@
+name: Java CI
+
+on: [push, pull_request]
+
+jobs:
+  build:
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        # windows-latest currently fails on some tests
+        os: [ubuntu-latest, macOS-latest, windows-latest]
+        java: [ '8', '11' ]
+      fail-fast: false
+    name: JAVA ${{ matrix.java }} OS ${{ matrix.os }}
+    steps:
+    - uses: actions/checkout@v1
+    - name: Set up JDK
+      uses: actions/setup-java@v1
+      with:
+        java-version: ${{ matrix.java }}
+    - name: Build with Gradle
+      run: gradle build --stacktrace