Update continuous-integration.yml

Author: neilime

.github/workflows/continuous-integration.yml

@@ -15,7 +15,7 @@ jobs:
     strategy:
       matrix:
         operating-system: [ubuntu-latest]
-        php-versions: ["7.3", "7.4"]
+        php-versions: ["7.3", "7.4", "8.0"]
     runs-on: ${{ matrix.operating-system }}
     steps:
       - name: Checkout