Add CI job that validates the Gradle wrapper

See https://github.com/marketplace/actions/gradle-wrapper-validation

Author: bwaldvogel

.github/workflows/gradle.yml

@@ -22,6 +22,13 @@ jobs:
     - name: Build with Gradle
       run: ./gradlew build
 
+  validateGradleWrapper:
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - uses: gradle/[email protected]
+
   publishCoverage:
     runs-on: ubuntu-latest