Add more java versions to test matrix (#13)

Author: dampcake

.github/workflows/ci.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        java: [ 8, 9, 10, 11, 12, 13, 14, 15 ]
+        java: [ 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18 ]
     env:
       JAVA: ${{ matrix.java }}
     name: Java ${{ matrix.java }}