Change from deprecated macOS-10.14 to macOS-latest in GitHub Workflow

.github/workflows/ci.yml

@@ -4,7 +4,7 @@ jobs:
   makepanda:
     strategy:
       matrix:
-        os: [ubuntu-16.04, windows-2016, macOS-10.14]
+        os: [ubuntu-16.04, windows-2016, macOS-latest]
     runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v1