Update composer.json

Author: jakewhiteley

composer.json

@@ -7,7 +7,7 @@
         "php": "^5.4 || ^7.0"
     },
     "autoload": {
-        "files": "functions.php",
+        "files": ["functions.php"],
         "psr-4": {
             "JakeWhiteley\\PhpSets\\": ""
         }