GH Actions: don't fail fast on "low" PHP versions

I'd like to suggest adding this as a temporary measure until the failing code has been fixed.

The `fail-fast` setting defaults to `true` and has the effect of cancelling any and all running/waiting builds within the same workflow as soon as one build has failed.

By setting this to `false`, all builds will be always be run, allowing for easier debugging of the current test failures.

.github/workflows/ci.yml

@@ -7,6 +7,7 @@ jobs:
     name: PHP ${{ matrix.php-versions }} Test on ${{ matrix.operating-system }}
     runs-on: ${{ matrix.operating-system }}
     strategy:
+      fail-fast: false
       matrix:
         operating-system: ['ubuntu-18.04']
         php-versions: ['5.3', '5.4', '5.5', '5.6', '7.0']