GitHub actions (#646)

* Added github action to test the build on each push

On each push gradlew build will be executed on Ubuntu 18.04 using JDK 8, 11.

.github/workflows/ci.yml

@@ -0,0 +1,28 @@
+---
+name: Java CI
+
+on: [push]
+
+jobs:
+  test:
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [ubuntu-18.04]
+        java: [8, 11]
+      fail-fast: false
+      max-parallel: 4
+    name: Test JDK ${{ matrix.java }}, ${{ matrix.os }}
+
+    steps:
+      - uses: actions/checkout@v2
+      - name: Set up JDK
+        uses: actions/setup-java@v1
+        with:
+          java-version: ${{ matrix.java }}
+      - name: Grant execute permission for gradlew
+        run: chmod +x gradlew
+      - name: Build with Gradle
+        run: ./gradlew build
+
+...