Also build and test on JDK 15

Author: bwaldvogel

.github/workflows/gradle.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        java: [ '8', '11', '14' ]
+        java: [ '8', '11', '14', '15' ]
 
     steps:
     - uses: actions/checkout@v2