GitHub Actions upgrade to actions/checkout@v2

Author: nhojpatrick

.github/workflows/ci.yaml

@@ -15,7 +15,7 @@ jobs:
     name: Test JDK ${{ matrix.java }}, ${{ matrix.os }}
 
     steps:
-      - uses: actions/checkout@v1
+      - uses: actions/checkout@v2
       - name: Set up JDK
         uses: actions/setup-java@v1
         with: