Fix for missing JFX with GitHub Actions (#793)

* Possible fix for missing JFX

See actions/setup-java#18 and docs for setup-java.

* Update gradle.yml

.github/workflows/gradle.yml

@@ -20,6 +20,7 @@ jobs:
       uses: actions/setup-java@v1
       with:
         java-version: 1.8
+        java-package: jdk+fx
     - name: Grant execute permission for gradlew
       run: chmod +x gradlew
     - name: Build with Gradle