chore: add JDK 13 to build matrix

Author: dmaccormack

.github/workflows/ci.yml

@@ -43,7 +43,7 @@ jobs:
     strategy:
       matrix:
         os: [macos-latest, ubuntu-latest, windows-latest]
-        jdk: [8, 9, 10, 11, 12]
+        jdk: [8, 9, 10, 11, 12, 13]
     steps:
     - uses: actions/checkout@v2
     - name: Set up JDK  ${{ matrix.jdk }}