Add default composer-json, overwrite for PHP7.4

- have to specify mariadb-version and extension for PHP 7.4
  at the same time to prevent a 2nd GitHub job being created.

.github/workflows/continuous-integration.yml

@@ -16,18 +16,11 @@ jobs:
         php-version: ["5.4", "5.5", "5.6", "7.0", "7.1", "7.4"]
         mariadb-version: ["10.3"]
         extension: ["pdo_mysql"]
+        composer-json: ["composer.json"]
         include:
-          - php-version: "5.4"
-            composer-json: "composer.json"
-          - php-version: "5.5"
-            composer-json: "composer.json"
-          - php-version: "5.6"
-            composer-json: "composer.json"
-          - php-version: "7.0"
-            composer-json: "composer.json"
-          - php-version: "7.1"
-            composer-json: "composer.json"
           - php-version: "7.4"
+            mariadb-version: "10.3"
+            extension: "pdo_mysql"
             composer-json: "composer-7.4.json"
 
     services: