GitHub actions add ubuntu-20.04

Author: nhojpatrick

.github/workflows/ci.yaml

@@ -8,7 +8,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-18.04]
+        os: [ubuntu-18.04, ubuntu-20.04]
         java: [7, 8, 11, 17]
       fail-fast: false
       max-parallel: 4