Use JDK 11

Closes gh-21

.github/workflows/continuous-integraion-workflow.yml

@@ -16,7 +16,7 @@ jobs:
       - name: Set up JDK
         uses: actions/setup-java@v1
         with:
-          java-version: '8'
+          java-version: '11'
       - name: Cache Gradle packages
         uses: actions/cache@v2
         with: