GH Actions: ubuntu-16.04 is no longer supported

... use `ubuntu-18.04` or `ubuntu-latest` for `20.04` instead.

Also see:
* https://ubuntu.com/blog/ubuntu-16-04-lts-transitions-to-extended-security-maintenance-esm
* shivammathur/setup-php#452

.github/workflows/ci.yml

@@ -8,7 +8,7 @@ jobs:
     runs-on: ${{ matrix.operating-system }}
     strategy:
       matrix:
-        operating-system: ['ubuntu-16.04']
+        operating-system: ['ubuntu-18.04']
         php-versions: ['5.3', '5.4', '5.5', '5.6', '7.0']
         phpunit-versions: ['7.5.20']
     steps: