Upgraded to Gradle 6.6 and Kotlin 1.4.0.

.github/workflows/gradle.yml

@@ -0,0 +1,21 @@
+name: Java CI with Gradle
+
+on: [push, pull_request, workflow_dispatch]
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Set up JDK 1.8
+      uses: actions/setup-java@v1
+      with:
+        java-version: 1.8
+    - name: Grant execute permission for gradlew
+      run: chmod +x gradlew
+    - name: Test with Gradle
+      run: ./gradlew check
+      env:
+        PINBOARD_API_TOKEN: ${{ secrets.PINBOARD_API_TOKEN }}