add Gradle wrapper validation

Author: sullis

.github/workflows/ci.yml

@@ -16,6 +16,7 @@ jobs:
         java: [ '8', '11' ]
     steps:
     - uses: actions/checkout@v2
+    - uses: gradle/wrapper-validation-action@v1
     - name: Set up JDK
       uses: actions/setup-java@v1
       with: